The membranes surrounding the ripe ovarian, ovum are two: (1) a single outer, thick, zona radiata, with a granular peripheral and a transparent inner portion, pierced radially by fine canals through which nutriment is obtained by the ovum from, the follicular cells (of the discus proligerus) immediately in contact with the zona: (2) an inner very delicate vitelline membrane which closely covers the ovum itself; and between these membranes is a space, the circum-vitelline space. The confirmation of Reichert's (No. 18), Meyer's (No. 17), and van Beneden's (No. 4) observations as to the presence of the inner delicate vitelline membrane appears of some interest as many embryologists are still sceptical of its existence, while the relation of the follicular cells with the radial canals of the zona supports the view as to the source of the nutriment of the ovarian ovum. On the other hand the fact that nothing was seen comparable to a micropyle in the zona, such as M. Barry (No. 3), and Meissner (No. 16), described, nor any follicular cells within the zona such as Lindgren (No. 15), von Sehlen (No. 21), and Virchow (No. 22), have observed, is some further proof that the conditions of the material investigated by these authors was abnormal.
The yolk contained within the ovum, which is of two kinds: viz. (1) homogeneous vesicular bodies, (2) minute highly refractile granules, is contained within the meshes of a protoplasmic reticulum; it is dense and contains no large globules such as Beneden (Nos. 6 and 7) describes in theBat's ova. The rounded or oval nucleus contains a single centrally placed nucleolus and a variable number of smaller or larger granules, which may possibly be considered as nucleolar material. During maturation the vitellus becomes divided into a medullary granular, and a cortical non-granular portion, the circum-vitelline space between the zona radiata and the vitelline membrane is enlarged, while the vitellus itself contracts away from the vitelline membrane excepting (1) here and there where pseudopodia-like processes connect the two, and (2) at one spot where the polar bodies are formed. At this latter place two polar bodies may be seen in the specimen figured, outside the vitelline membrane, whilst the nucleus remains as the female pronucleus lying in the peripheral portion of the ovum. Finally, the vitellus again expands and the nucleus retires to the centre of the ovum and is no longer to be seen. Assuming that these observations are correct, Beneden's description of the ejection of the vesicle to form the polar bodies and the subsequent non-nucleated condition of the ovum must be considered erroneous.
Impregnation appears to be effected by a single spermatazoon, although a considerable number of spermatazoa find their way through the zona and may be seen lying passively in the circumvitelline space.
The segmentation occurs while the ovum travels down the Fallopian tube. Two and then four segments are formed, after which the course of segmentation is irregular. The segments themselves are of irregular size and do not appear to be divisible into two kinds (epiblastic and hypoblastic) as Beneden describes. After its entrance into the uterus, a division of the segments into an outer hyaline layer and inner deeply granular mass takes place, and I would suggest the hypothesis that the vitelline matter which was originally contained in all segments alike has been transmitted from the outer segments to the segments lying in the interior of the ovum, in order that the former segments may the more readily and actively multiply and flatten out to form the wall of the blastodermic vesicle. The epiblast of the vesicle and of the embryo is derived from the whole of the outer layer and by far the largest proportion of the inner mass of segments. The hypoblast is derived from the small remaining portion of the inner mass and the mesablast, subsequently, from both epiblast and hypoblast layers. This being the case, the division of the segmentation spheres, by Beneden, into epiblast and hypoblast spheres from the time when the first two segments were formed, is incorrect; and at the same time the theory of a comparison of the metagastrula stage with the gastrula of other animals is likewise untenable.
Downregulation of Extraembryonic Tension Controls Body Axis Formation in Avian Embryos
