An investigation has been made of the junction between the shell and cap in the egg-shell of Rhodnius prolixus. This complex region consists of the thickened rim of the cap connected by a thin sealing bar to the rim of the shell. The secretion of this part of the shell has been followed and compared with the formation of less specialized portions of the shell.
The shell has been divided into units, each the product of an individual follicle cell. It has been found that all the seven layers which make up the unspecialized parts of the shell are present in the seal complex; that these consist of five endochorion layers and two exochorion layers in their normal order.
The exochorion is secreted around long villi, one from each follicle cell. These give rise to follicular pits in the shell.
In this complex region, cells start to secrete at various stages in the seven-phase cycle; their initial secretion is apparently related to the material with which they make contact at that time. After secretion has started, each cell completes the remainder of the cycle.
The rim of the cap is the product of four rings of follicle cells; the additional thickness is achieved by an increase in the exochorion layers, secreted around a series of very long follicular pits.
The sealing bar, which is produced by one ring of follicle cells, is composed of the inner four layers of the chorion only; the cells do not produce soft endochorion, or exochorion layers.
At the cap end of the sealing bar there is the predetermined hatching line. It is apparently produced by the presence in the follicle of cells which are inactive during the secretion of the inner layers, and so prevent co-ordination between the active cells on either side. A weak point is also present at the base of the sealing bar, at the site of other inactive cells, though this fissure is not used at hatching.
The rim of the shell is similarly produced by an expansion of the exochorion layers secreted around four rings of follicular villi. Of these, three rings of pits are filled in towards the end of secretion, but the fourth, lying on the upper portion of the rim, remains. These pits become the micropyles and associated structures.
There are 200 pits in the completed rim, divided into two groups. About fifteen are micropyles; the remainder are cavities closed at each end, and to which the name ‘pseudomicropyle’ has been given.
The pseudomicropyles are formed in a similar way to normal follicular pits, but start in the resistant protein layer, 0.5µ from the inside of the shell. They end in the resistant exochorion, where they are connected to the external surface by small bunches of pore canals. They probably play some part in the respiration of the embryo.
The true micropyles form the only free path through the shell. The inner portion of each tube is lined with hydrophilic protein, and the outer portion, which lies slightly posterior to the pseudomicropyles, is composed of hydrophobic lipoprotein.
The number of true micropyles is not constant, there being between ten and twenty scattered irregularly around the rim. However, eggs produced by older females contain fewer micropyles; this may account for a higher rate of sterility among these eggs.
The cells which form the micropyles and pseudomicropyles are the only ones which do not adhere to the typical cycle of seven secretory products. But in omitting three phases, the attachment of the exochorion to a protein layer is retained.
Evidence suggests that the cells forming the micropyles are determined in the earliest stages of secretion by being squeezed out of the pseudomicropylar ring of cells.
The epicuticle in an insect,
Rhodnius prolixus
(Hemiptera)
