scholarly journals SUBMICROSCOPIC TRANSFORMATIONS OF THE HYPOTHALAMUS STRUCTURE UNDER CONDITIONS OF 24-HOUR LIGHTING

2020 ◽  
Vol 19 (4) ◽  
pp. 5-9
Author(s):  
R. Bulyk ◽  
K. Vlasova
Keyword(s):  
Pineal Gland ◽  
Functional Activity ◽  
Ganglion Cells ◽  
Secretory Granules ◽  
Light Stress ◽  
Neurosecretory Cells ◽  
Free Access ◽  
Dense Matrix ◽  
White Rats ◽  
Supraoptic Nuclei

Light information that is perceived by the retinal photoreceptors is transmitted along the retinohypothalamic pathway (the retinal ganglion cells present in it) and supraoptic fibers, suprachiasmatic, paraventricular, arcuate nuclei of the hypothalamus, through the trunk of the upper thoracic region and lateral intermedial nuclei of the spinal cord, sym- pathic neurons of the superior cervical ganglion in the pineal gland. In the dark, signals from the suprachiasmatic nucleus enhance the synthesis and release of norepinephrine from sympathetic endings. The experiments were carried out on 40 sexually mature male outbred white rats weighing 0.15-0.18 kg. The animals were kept in cages at a constant temperature, air humidity and free access to water and food. Experimental animals were divided into two series, in each of which the biomaterial was taken at 14.00 and 02.00 h. The selected timing of the experiment is due to the different functional activity of the pineal gland in the indicated time periods of the day. Submicroscopic studies of the supraoptic nuclei of the hypothalamus of intact animals at 14:00 showed that the majority of neurosecretory cells are round-oval in shape with rare invaginations and nuclei of irregular shape with shallow invaginations of the karyolem. The karyoplasm contains lumps of chromatin and a dense osmiophilic nucleolus. Neuroplasm occupies a small volume, in it the tubules of the granular endoplasmic reticulum are tightly packed with a small lumen, where there are many ribosomes and polysomes, as well as a small amount of evenly distributed granules. In the Golgi complex, secretory granules of different sizes are formed. Mitochondria, small with a dense matrix, contain few cristae. Some of them are in an energetically stressed state, are able to hypertrophy and partially lose their cristae or die. The ultramicroscopic organization of neurosecretory cells of the supraoptic nuclei of the anterior hypothalamus of rats was investigated. Under the standard illumination mode (12.00C: 12.00T), the ultrastructure of neurons indicates a decrease in their functional activity during the light period of the day and growth - in the dark period of the day. Light stress (24.00C: 00T) leads to significant desynchronosis and causes destructive changes in the components of the structures under study, more pronounced at 02.00 h.

scholarly journals The influence of melatonin on stress-induced ultramicroscopic changes of the neurons of the supraoptic nuclei of the rat hypothalamus

2021 ◽  
Vol 25 (3 (99)) ◽  
pp. 25-32
Author(s):  
R. Bulyk ◽  
T. Bulyk ◽  
O. Smetanuik
Keyword(s):  
Immobilization Stress ◽  
Prolonged Exposure ◽  
Neurosecretory Cells ◽  
Ultrastructural Organization ◽  
Neurosecretory Granules ◽  
Electron Microscopic ◽  
Relative Improvement ◽  
White Rats ◽  
Hypothalamic Nuclei ◽  
Supraoptic Nuclei

The aim: to study the effect of melatonin on the ultrastructural state of the supraoptic nuclei of the hypothalamus of rats under immobilization stress.Materials and methods. The experiments were performed on non-linear male white rats weighing 200-220 g. The animals were divided into 3 study series, in each of which the biomaterial was collected at 2 p.m. and at 2 a.m. using electron microscopic method. Long immobilization stress was simulated by keeping rats in special plastic penal cages for 6 hours daily for 7 consecutive days. Melatonin (Sigma, USA, 99.5% purification degree) at a dose of 0.5 mg/kg, in 1.0 ml of solvent (0.9% ethanol solution on physiologic saline) was injected daily, intraperitoneally.Results. When the animals were kept under the standard light regime, the ultrastructural organization of the hypothalamic nuclei at 2 p.m. indicated their low functional activity in comparison with the studies carried out at 2 a.m. Prolonged exposure of rats to immobilization stress was reflected in a significant rearrangement of the ultrastructural organization of supraoptic nuclei of the hypothalamus. The established changes can be considered as a manifestation of neurosecretory activity suppression, a decrease in neurosecretase production by hypothalamic neurons. Melatonin injections against the background of immobilization stress resulted in relative normalization of ultrastructural state of neurons of supraoptic nuclei of the hypothalamus of animals. In particular, studies at 2 a.m. revealed light neurosecretory cells containing a large nucleus, it was pyknotically altered. Karyolema invaginations, euchromatin dominance in the nucleus were observed. Heterogeneous changes were observed on the part of mitochondria. Enlarged tubules of granular endoplasmic reticulum were seen. At the same time, a small number of ribosomes and few hormonal granules were noticeable in neuroplasm. The mentioned picture of neurosecretory cells reflects a relative improvement in their electron microscopic state, which is evidenced by the appearance of neurosecretory granules. However, the ultrastructure of other organelles of the studied neurons indicates a depleted state caused by prolonged immobilization.Conclusions. 1. In animals under standard photoperiod conditions, the structural organization of supraoptic neurons of the hypothalamic nuclei during the nighttime of the experiment reflects the intensity of intracellular synthesizing processes (at 2 a.m.). A decrease in the activity of the structures under study is noted during the daytime. 2. Under immobilization stress, the ultrastructural organization of the above neurons indicates a pronounced disturbance of reactive nature with the signs of decreased functional ability of the structures and the phenomena of edema and destruction during the period of observation. 3. Melatonin injections against the background of immobilization stress led to a relative improvement in the ultrastructural state of the animals’ hypothalamic nuclei neurons, which is evidenced by the appearance of neurosecretory granules. However, the ultrastructure of other organelles of the studied neurons indicated a depleted state caused by prolonged immobilization.

scholarly journals Ultrastructural Changes in Pineal Cells of the Epiphysis during Long Exposure of Drinking Water Nitrates and Correction Means

2021 ◽  
Vol 6 (2) ◽  
pp. 249-257
Author(s):  
A. Yu. Chumachenko ◽  
◽  
Е. G. Redka ◽  
Keyword(s):  
Methylene Blue ◽  
Pineal Gland ◽  
Functional Activity ◽  
Secretory Granules ◽  
Ultrastructural Level ◽  
The Body ◽  
Simultaneous Action ◽  
Type I ◽  
Stress Reactions ◽  
Old Rats

According to modern data, the epiphyse (pineal gland) is an organ that combines the processes of adaptogenesis and immunogenesis, takes part in triggering stress reactions and determines the sequence of disorders in the body at different stages of stress development. Researchers consider the pineal gland to be the most important organizer of biological rhythms associated with photoperiodism and the organ that determines the stereotype of the organism. Its individual functions are rhythmically variable under the influence of the environment and age. The purpose of the study was to study the structural and functional changes of pineal cells of the pineal gland in rats at different stages of normal development, under the action of nitrates and the simultaneous action of nitrates and methylene blue. Materials and methods. In accordance with the purpose of the work, the study was carried out on 90 nonlinear white male rats of different ages. The animals were kept in the vivarium in equivalent conditions. Long-term exposure to nitrates on the body of animals was achieved by daily introduction into the drinking ration, starting from the 7th day of postnatal development of rats (after preliminary water purification), 120 mg/l sodium nitrate, that is, in a dose that is typical for many regions of Ukraine. When simulating the action of methylene blue, this substance was daily orally administered to the animals in doses: 0.1-0.15 ml of a 1% aqueous solution per 1 kg of body weight. Results and discussion. As a result of the 7-day action of nitrates in 14-day-old rats, structural changes were observed in the pineal gland, which corresponded to a decrease in the function of light cells and an increase in the functional activity of type II pinealocytes. The ultrastructure of the cytoplasm of type I pinealocytes contained poorly developed organelles and single secretory granules. In 45-day-old animals exposed to nitrates in light pinealocytes, pronounced disturbances in membrane organelles, primarily in mitochondria and the granular endoplasmic reticulum, were noted. The functional activity of dark pinealocytes increased during this period of the study. In the pineal parenchyma of 90-day-old rats after exposure to nitrates, the functional activity of type I pinealocytes was at a low level. The functional activity of dark pinealocytes was also weakened. Thus, as a result of the simultaneous action of nitrates and methylene blue in the pineal gland of 14-day-old rats, a tendency to gradual restoration of the structural and functional parameters of cells was observed. In 45-day-old animals, after the simultaneous action of nitrates and methylene blue, the ultrastructural data of pineal cells indicated numerous mitochondria and secretory granules in the cytoplasm. In the parenchyma of the pineal gland of 90-day-old rats after chronic action of nitrates and methylene blue at the ultrastructural level, no sharp changes in the cytoplasm and nucleus of light and dark pinealocytes were found in comparison with the control. Conclusion. The intake of nitrates in 14-day-old rats causes the development of a stress reaction, poorly developed organelles and signs of degranulation appear in the ultrastructure of light pinealocytes, however, the cytoplasm and nuclei of dark cells indicated an increase in function. In 45-day-old rats after exposure to nitrates, the signs of the stress reaction are enhanced. In the ultrastructure of the cytoplasm of light cells, pronounced violations of membrane organelles are determined. Enhanced function continues in dark pinealocytes. After the action of nitrates in 90-day-old rats, changes occur that are characteristic of the stage of depletion of the general adaptation syndrome, the result of which is a deep imbalance in the work of the pineal gland. The combined action of nitrates and methylene blue in 14-day-old animals helps to reduce the toxic effect and the strength of stress reactions in the pineal gland. In the ultrastructure of pinealocytes, the number of ribosomes, small secretory granules and mitochondria increases in comparison with the action of nitrates alone. In 45-day-old animals with the simultaneous intake of nitrates and methylene blue in the ultrastructure of melanotropic cells, the accumulation of secretory granules of the same size and electron density, an increase in the number of organelles and signs of restoration of the structure of the cytoplasm and nucleus are noted. The use of methylene blue against the background of long-term intake of nitrates in 90-day-old rats at the ultrastructural level of abrupt changes in the cytoplasm and nucleus of light and dark pinealocytes is not manifested in comparison with the control

The cytoskeleton as a barrier to exocytosis in secretory cells

1988 ◽  
Vol 139 (1) ◽  
pp. 253-266 ◽  
Author(s):  
D. Aunis ◽  
M. F. Bader
Keyword(s):  
Chromaffin Cells ◽  
Binding Proteins ◽  
Secretory Granules ◽  
Actin Binding ◽  
Bacterial Toxin ◽  
Secretory Cells ◽  
Free Access ◽  
Actin Binding Proteins ◽  
Permeabilized Cells ◽  
Cytoskeletal Network

Chromaffin cells of the adrenal medulla synthesize, store and secrete catecholamines. These cells contain numerous electron-dense secretory granules which discharge their contents into the extracellular space by exocytosis. The subplasmalemmal area of the chromaffin cell is characterized by the presence of a highly organized cytoskeletal network. F-Actin seems to be exclusively localized in this area and together with specific actin-binding proteins forms a dense viscoelastic gel; fodrin, vinculin and caldesmon, three actin cross-linking proteins, and gelsolin, an actin-severing protein, are found in this subplasmalemmal region. Since fodrin-, caldesmon- and alpha-actinin-binding sites exist on secretory granule membranes, actin filaments can also link secretory granules. Chromaffin granules can be entrapped in this subplasmalemmal lattice and thus the cytoskeleton acts as a barrier preventing exocytosis. When cells are stimulated, molecular rearrangements of the subplasmalemmal cytoskeleton take place: F-actin depolymerizes and fodrin reorganizes into patches. In addition, introduction of monospecific antifodrin immunoglobulins into digitonin-permeabilized cells blocks exocytosis, demonstrating the crucial role of this actin-binding protein. In bacterial toxin-permeabilized chromaffin cells, experiments using actin-perturbing agents such as cytochalasin D and DNAase I suggest that exocytosis is in part controlled by the cytoskeleton. The intracellular signal governing the cytoskeletal reorganization (associated with exocytosis) is calcium. Calcium inhibits some and activates other actin-binding proteins and consequently causes dissolution of the subplasmalemmal cytoskeleton. This dissolution of cytoskeletal filaments should result in granule detachment and permit granules free access to exocytotic sites on the plasma membrane.

C-type Natriuretic Peptide Inhibits L-type Ca2+ Current in Rat Magnocellular Neurosecretory Cells by Activating the NPR-C Receptor

2005 ◽  
Vol 94 (1) ◽  
pp. 612-621 ◽  
Author(s):  
Robert A. Rose ◽  
Madhu B. Anand-Srivastava ◽  
Wayne R. Giles ◽  
Jaideep S. Bains
Keyword(s):  
Natriuretic Peptide ◽  
Action Potentials ◽  
Hormone Secretion ◽  
Inhibitory Effect ◽  
Neurosecretory Cells ◽  
Mediated Effects ◽  
Inhibitory G Protein ◽  
Whole Cell Recordings ◽  
Supraoptic Nuclei

Magnocellular neurosecretory cells (MNCs), of the paraventricular and supraoptic nuclei of the hypothalamus, secrete the hormones vasopressin and oxytocin. As a result, they have an essential role in fundamental physiological responses including regulation of blood volume and fluid homeostasis. C-type natriuretic peptide (CNP) is present at high levels in the hypothalamus. Although CNP is known to decrease hormone secretion from MNCs, no studies have examined the role of the natriuretic peptide C receptor (NPR-C) in these neurons. In this study, whole cell recordings from acutely isolated MNCs, and MNCs in a coronal slice preparation, show that CNP (2 × 10−8 M) and the selective NPR-C agonist, cANF (2 × 10−8 M), significantly inhibit L-type Ca2+ current ( ICa(L)) by ∼50%. This effect on ICa(L) is mimicked by dialyzing a Gi-activator peptide (10−7 M) into these cells, implicating a role for the inhibitory G protein, Gi. These NPR-C–mediated effects were specific to ICa(L). T-type Ca2+ channels were unaffected by CNP. Current-clamp experiments revealed the ability of CNP, acting via the NPR-C receptor, to decrease (∼25%) the number of action potentials elicited during a 500 ms depolarizing stimulus. Analysis of action potential duration revealed that CNP and cANF significantly decreased 50% repolarization time (APD50) in MNCs. In summary, our findings show that CNP has a potent and selective inhibitory effect on ICa(L) and on excitability in MNCs that is mediated by the NPR-C receptor. These data represent the first electrophysiological evidence of a functional role for the NPR-C receptor in the mammalian hypothalamus.

scholarly journals Detection of nitric oxide synthase (NOS) in somatostatin-producing cells of human and murine stomach and pancreas.

1996 ◽  
Vol 44 (4) ◽  
pp. 339-346 ◽  
Author(s):  
M A Burrell ◽  
L M Montuenga ◽  
M García ◽  
A C Villaro
Keyword(s):  
Nitric Oxide ◽  
Nitric Oxide Synthase ◽  
Ganglion Cells ◽  
Secretory Granules ◽  
The Body ◽  
Gastric Epithelium ◽  
Thin Sections ◽  
Immunocytochemical Techniques ◽  
D Cells ◽  
Oxide Synthase

The aim of this study was to identify by immunocytochemistry the distribution of nitric oxide synthase (NOS) in human and murine gastric epithelium. Using two different antisera specific for neuronal NOS (nNOS), we detected nNOS immunoreactivity in endocrine cells of the epithelium of the body and pyloric regions as well as in ganglion cells of the intrinsic plexi of the stomach of the three species studied. Both immunocytochemistry of contiguous sections and double immunolabeling methods showed that the nNOS-immunoreactive cells were also immunoreactive for somatostatin. Co-localization of nNOS and somatostatin has also been found in the pancreatic islets, where strong nNOS immunoreactivity appeared in scattered cells, which were peripheral in rat and mouse islets and more randomly distributed in human. The possibility of crossreactivity between the antisera against nNOS and somatostatin was ruled out by means of absorption controls. Immunocytochemical techniques were also applied to thin sections, confirming the immunostaining of gastric D-cells, which was restricted principally to the secretory granules. The possible functional implications of these findings for gastric and pancreatic physiology are discussed.

scholarly journals Ca2+-dependent and -independent release of neurotransmitters from PC12 cells: a role for protein kinase C activation?

1984 ◽  
Vol 99 (2) ◽  
pp. 628-638 ◽  
Author(s):  
T Pozzan ◽  
G Gatti ◽  
N Dozio ◽  
L M Vicentini ◽  
J Meldolesi
Keyword(s):  
Protein Kinase C ◽  
Protein Kinase ◽  
Secretory Granules ◽  
Neurosecretory Cells ◽  
Cell Ultrastructure ◽  
Marked Rise ◽  
Kinase C ◽  
Secretion Granules ◽  
Granule Structure ◽  
Protein Kinase C Activation

The intracellular mechanisms regulating the process of evoked neurotransmitter release were studied in the cloned neurosecretory cell line PC12. Various agents were employed that were known, from previous studies in other systems, to stimulate release in a manner either strictly dependent or independent of the concentration of extracellular Ca2+, [Ca2+]o. Three parameters were investigated in cells suspended in either Ca2+-containing or Ca2+-free Krebs-Ringer media: release of previously accumulated [3H]dopamine; average free cytoplasmic Ca2+ concentration, [Ca2+]i (measured by the quin2 technique); and cell ultrastructure, with special reference to the number and structure of secretion granules. The release induced by the ionophores transporting monovalent cations, X537A and monensin, occurred concomitantly with profound alterations of secretory granule structure (swelling and dissolution of the dense core). These results suggest that the effect of these drugs is due primarily to leakage of dopamine from granules to the cytoplasm and extracellular space. In contrast, the changes induced by other stimulatory drugs used concerned not the structure but the number of secretory granules, indicating that with these drugs stimulation of exocytosis is the phenomenon underlying the increased transmitter release. The release response induced by the Ca2+-ionophore ionomycin was dependent on [Ca2+]o, occurred rapidly, was concomitant with a marked rise of [Ca2+]i, and ceased after 1-2 min even though [Ca2+]i remained elevated for many minutes. 12-O-tetradecanoylphorbol, 13-acetate and diacylglycerol (both of which are known as activators of protein kinase C) induced slow responses almost completely independent of [Ca2+]o and not accompanied by changes of [Ca2+]i. Combination of an activator of protein kinase C with a low concentration of ionomycin failed to modify the [Ca2+]i rise induced by the ionophore, but elicited a marked potentiation of the release response, which was two- to fourfold larger than the sum of the responses elicited separately by either drugs. Thus, activation of protein kinase C seems to play an important role in the regulation of exocytosis in neurosecretory cells, possibly by increasing and maintaining the sensitivity to Ca2+ of the intracellular apparatus regulating granule discharge by exocytosis.

scholarly journals Ultrastructural Changes of Melanotropic Cells of the Intermediate Share of the Adenogophysis in the Age Aspect

2021 ◽  
Vol 6 (1) ◽  
pp. 339-344
Author(s):  
A. Y. Chumachenko ◽  
◽  
A. G. Redka ◽  
Keyword(s):  
Pituitary Gland ◽  
Functional Activity ◽  
Secretory Granules ◽  
Male Rats ◽  
Ultrastructural Changes ◽  
Glandular Cell ◽  
Intermediate Lobe ◽  
Different Ages ◽  
Old Rats ◽  
Intact Rats

In modern theoretical and practical biology and medicine, the key problem of research is to reveal the patterns of structural and functional organization of the human and animal body at different stages of development. Literature sources provide very limited data on the organization of the intermediate lobe of the adenohypophysis in humans, leaving insufficiently studied the ultrastructural state and activity of melanotropic cells of the intermediate lobe of the adenohypophysis in animals of different ages, including rats. The issues of the intermediate pituitary gland functioning remain important and little studied, especially in the period of the beginning of the optic-thalamic system. The purpose of the research was to study the ultrastructural changes of melanotropic cells of the intermediate lobe of the adenohypophysis in rats of different ages in the norm. Material and methods. In accordance with the purpose of the study we conducted the experiment on 30 nonlinear white male rats of different ages: 14-, 45- and 90-day-old. The animals were kept in the vivarium in equivalent conditions. The keeping and using of animals was carried out in accordance with the provisions of the "General Ethical Principles of Animal Experiments", approved by the IV National Congress of Bioethics. While examining the intermediate pituitary gland of intact rats on the electron microscope, the material was fixed in 2.5% solution of glutaraldehyde on phosphate buffer with fixation in 1% solution of osmium tetroxide according to Caulfield. It was dehydrated in alcohols of increasing concentration (70%, 80%, 90%, 100%) and acetone, poured into a mixture of epon-araldite. Semi-thin sections were made from the obtained blocks, which were stained with toluidine blue. Results and discussion. At the ultrastructural level in the intermediate lobe of the adenohypophysis of 14-day-old intact rats, several cell types could be identified that differed in the number and size of secretory granules. The ultrastructure of glandular cells of 45-day-old rats had no significant differences compared with 14-day-old animals. Secretory granules of different sizes and electron densities were observed in the cytoplasm of melanotropic cells. It was often possible to see a glandular cell with numerous secretory granules in one part of the cytoplasm, while in another they were virtually absent. Most melanotropocytes were characterized by slightly compacted mitochondria, but their numbers were slightly higher than in 45-day-old rats. The nuclei of most cells were large, oval in shape with a clear structure of nucleoli and their ribosomal component. The latter represented groups of ribosomes that were collected in osmophilic complexes. The amount of heterochromatin exceeded euchromatin and it was located mainly in the membrane with areas of rarefaction in the pore area Conclusion. In 14-day-old intact male rats, the intermediate lobe of the adenohypophysis was presented as a formed functionally active organ. In the ultrastructure of the cytoplasm of melanotropes there was a moderate development of organelles, and judging by the number and size of secretory granules, as well as the density of their content, we can assume that all these cells differed from each other in their functional activity. In 45-day-old intact rats, accumulation of secretory granules was observed in the cytoplasm of melanotropes, especially near the nucleus, which indicated an increase in melanocyte-stimulating hormones synthesis with age. The ultrastructural state of the cytoplasm and nucleus also indicated an increase in functional activity. The ultrastructure of the intermediate lobe cells of the adenohypophysis of rats at the age of 90 days differed from that of younger animals by signs of different functional activity of individual melanotropic cells

scholarly journals Cell-Subtype-Specific Remodeling of Intrinsically Photosensitive Retinal Ganglion Cells in Streptozotocin-Induced Diabetic Mice

2021 ◽  
Author(s):  
Wei-Yi Chen ◽  
Xu Han ◽  
Ling-Jie Cui ◽  
Chen-Xi Yu ◽  
Wen-Long Sheng ◽  
...  
Keyword(s):  
Neurodegenerative Diseases ◽  
Retinal Ganglion Cells ◽  
Visual Function ◽  
Functional Activity ◽  
Ganglion Cells ◽  
Early Stage ◽  
Retinal Ganglion ◽  
Diabetic Mice ◽  
Pupillary Light ◽  
Dendritic Branching

Recent evidence suggests that melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs), a neuronal class regulating non-image forming (NIF) vision and generally thought to be injury-resistant, are dysfunctional in certain neurodegenerative diseases. Although disrupted NIF visual functions have been reported in patients and animals with diabetes, it remains controversial whether ipRGCs exhibit remodeling during diabetes and if so, whether such remodeling is variable among ipRGC subtypes. Here we demonstrate that survival, soma-dendritic profiles and melanopsin-based functional activity of M1 ipRGCs were unaltered in streptozotocin-induced 3-month diabetic mice. Such resistance remained at 6 months after streptozotocin administration. In contrast, M2/M3 ipRGCs underwent significant remodeling in diabetic mice, manifested by enlarged somata and increased dendritic branching complexity. Consistent with the unaltered melanopsin levels, the sensitivity of melanopsin-based activity was unchanged in surviving M2 cells, but their response gain displayed a compensatory enhancement. Meanwhile, the pupillary light reflex, a NIF visual function controlled by M2 cells, was found to be impaired in diabetic animals. The resistance of M1 cells might be attributed to the adjacency of their dendrites to capillaries, which makes them less disturbed by the impaired retinal blood supply at the early stage of diabetes.

Memoirs: On Some Points in the Development of Petromyzon fluviatilis

1887 ◽  
Vol s2-27 (107) ◽  
pp. 325-377
Author(s):  
ARTHUR E. SHIPLEY
Keyword(s):  
Nervous System ◽  
Pineal Gland ◽  
Alimentary Canal ◽  
Ganglion Cells ◽  
Cranial Nerves ◽  
Middle Line ◽  
Seventh Nerve ◽  
Optic Vesicles ◽  
The Central Nervous System ◽  
Whole Muscle

I have now described the structure of the chief organs in my oldest larva, and I propose to conclude this paper by a brief summary of the results obtained. In the first place the mesoblast is not completed ventrally by a layer of cells split off from the hypoblastic yolk-cells, as Scott has described. But the ventral mesoblast is formed by the downgrowth of the mesoblastic plates, which ultimataly meet and unite in the ventral middle line. The blastopore does not close up, as later observers have maintained, but, as Max Schultze described thirty years ago, it persists as the anus. There is no neurenteric canal, though a solid strand of tissue proceeds back from the alimentary canal and fuses with an indifferentiated mass of cells, into which the nervous system and mesoblast also pass. The lumen of the alimentary canal is that of the mesenteron; it does not become obliterated during larval life. In its anterior end the hypoblast remains in connection with the epiblast at certain points, and here the gill-clefts arise; between these the mesoblast grows down and forms the gill-bars. The origin of the ciliated ring and the hypopharyngeal groove and hyperpharyngeal bar are also described, and the ciliated condition of the oesophagus and stomach. The "muscle-plates," whose structure is so peculiar in the Lamprey, arise each from a single cell of the mesoblastic somites. This increases in size, slides in between the neighbouring cells, and ultimately occupies the whole of the space between two myotomes. Its nucleus divides until each cell contains several nuclei. Striated fibrils then appear and increases till the whole "muscle-plate" consists of little else besides these fibrils, squeezing between them a few nuclei. These "muscle-plates" arise from the segmental half of the mesoblast; the muscles of the gills, lips, and probably of the eye, have a different structure and arise from the ventral unsegmented part. The blood-corpuscles arise from the ventral free edges of the mesoblast, before they unite in the ventral middle line, they collect in a large sinus just behind the heart. The heart appears in the ventral mesentery, formed by the union of the lateral mesoblastic plates; at first its lumen is continuous with the sinus just mentioned. This sinus lies between the hypoblastic yolk-cells and the epiblast; it subsequently acquires walls and forms part of the subintestinal vein. The ciliated funnels of the pronephros are left as apertures by the segmental duct which in its anterior end is formed from a groove. The groove closes up at intervals, leaving four or five openings which become the funnels. They do not arise as blind projections from the duct, which subsequently, acquire ciliated openings. From the first the pronephros has a double blood supply, pure blood from the aorta passing to the glomerulus, and impure blood in the cardinal veins surrounding the tubuli. The early development of the skeleton is described up to the stage where Professor Parker commenced his researches. The canal of the central nervous system developes after the neural chord has separated off from the epidermis; it does not appear to be lined by any invaginated epidermis, as Calberla and Scott maintained. The first sign of differentiation of the parts of the brain is the formation on the sixteenth day of the optic vesicles and pineal gland. The division into fore-, mid-, and hind-brain appears soon after, but the fore- and mid-brain are not separated by any well-marked groove. The first transverse commissure to appear is situated just in front of the stalk of the pineal gland. It forms the superior commissure of Osborn. Afterwards the ganglion cells thicken round it and form the asymmetrical ganglia habenulæ. The ganglia on the fifth, seventh, ninth, and tenth nerves are derived from epiblastic thickenings. Their roots probably arise as outgrowths from the neural ridge. The ganglion of the fifth divides into two parts, the ophthalmic and mandibular; these have a common root. The seventh nerve at its first appearance supplies the first or spiracular gill-cleft; when this is converted into the ciliated ring it continues to be supplied by the seventh nerve. The connection between the fifth, seventh, and tenth nerve ganglia does not exist and must be of later origin. The tenth nerve has a large ganglion on its root and bears a ganglion above each of the last six gill-clefts. No trace of the ram us lateralis is to be seen. The origin of the ganglia on the cranial nerves has no relation to the sense-organs of the skin; these have not appeared even in my oldest larva.

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