

mi 



THE EGG, FERTILIZATION, MATURATION, AND CLEAVAGE 25 



In some eggs the chorion appears to be homogeneous; in other cases it 

 may be more or less complex, either with an outer thicker exochorion 

 and a thin inner endochorion united by minute perpendicular trabeculae ; 

 or more or less felt-like with fibers running in all directions. Miller 

 (1939) states that the chorion of the stone-fly egg consists of two layers: 

 the exochorion, which is hard, thick, and lacunar and the very thin 

 endochorion, the two layers being separated by an interlamellar space 

 traversed by low cylindrical or discoid pillars, or trabeculae. 



Korschelt (1887) found that certain cells of the follicular epithelium, 

 the nuclei of which are not in alignment with those of adjacent cells and 

 lying in the region where the micro- 

 pylar orifices are to form, are pro- 

 vided distally with a pointed 

 protoplasmic process. After se- 

 cretion of the chorion by these "''" ^ ^^^^^ 

 cells at their distal end the with- .^^^^^ \ 

 drawal of the protoplasmic points 



c ,^ , ,-, . Fig. 26. — Micropyle formation. Sec- 



leaves passages tree that constitute ^^^^ ^j follicular epithelium (/oO and 



the micropylar canals (Fig. 26). In chorion of egg (c/i). (toi) Micropyle. (a-) 



the egg of the flesh fly Verhain ^^^^^ ^y^^- 



(1921) ascribes the formation of the micropyle not to a single cell but 

 to a group of follicular cells which push in from the extremity of the nurse 

 chamber between the nurse cells in the form of a cellular clavate process 

 and which probably form the micropylar core. The formation of the 

 micropylar canal in the Anopheles egg as described by Nicholson (1921) 

 in principle resembles that of the flesh fly. 



Pore canals in the chorion are similarly formed over the protoplasmic 

 processes of cells, but here the processes are, of course, much smaller. 



For the entrance of the sperms, micropjdar openings, either single or 

 in groups, are in most cases found at or near the anterior end of the egg. 

 But they may also be found lateral in position, as in some Orthoptera; 

 or a group of openings may be found at both ends, as in the eggs of the 

 flea ; or they may encircle the egg, as in the stone fly Pteronarcys. 



Immediately below the chorion is the vitelline membrane, a thin 

 noncellular tissue which Korschelt (1887) says is the hardened surface of 

 the periplasm and in some insects is formed before, in others after, the 

 formation of the chorion. Its elasticity, when it first appears, permits 

 it to conform with the changing shape of the developing egg. The mem- 

 brane is said to be lacking in some Collembola. According to Emeis 

 (1915) it is also apparently absent in some coccids, but Shinji (1919) states 

 that it is present in the species studied by him.. It should, however, not 

 be forgotten that the inner layers of the chorion may in some cases be 

 mistaken for the vitelline membrane (Slifer, 1937). 



