96 GENERAL EMBRYOLOGY 



is the micropyle, (Fig. 50), and, as we shall see, this sometimes 

 affords an aperture for the entrance of the sperm cell. 



A secondary membrane called the chorion often surrounds the 

 egg outside the vitelline (Figs. 42-50). This is a secretion 

 formed while the egg is still contained within the ovary, from 

 the cells there surrounding the egg, and its presence depends 

 upon the arrangement of these ovarian cells in the form of a 

 definite layer or epithelium surrounding the egg, termed the 



FIG. 50. A. Animal pole of the egg of the Cephalopod, Argonauta. From 

 Wilson, "Cell," after Ussow. Surrounding the egg is the chorionic membrane 

 perforated by the funnel-shaped micropyle, m. Beneath the micropyle lies the 

 egg nucleus in the cortical protoplasmic layer, p.b, polar bodies. B, C. Sec- 

 tions through the egg membranes and micropyle of the egg of the Teleosts Esox 

 (B) (ovarian egg) and Pygosteus (C) (ovarian egg, 0.4 mm. in diameter). After 

 Eigenmann. X 375. ez, zona radiata externa; /, egg follicle; iz, zona radiata 

 interna; m, micropylar cell; p, egg protoplasm; pp, protoplasmic processes; 

 y, yolk in egg; z, zona radiata. 



follicle. The chorion may be a thin flexible membrane, or a 

 tough resistant shell, as in Insects and Teleosts. Penetrating 

 the chorion there is nearly always a continuation of the micro- 

 pyle. The formation of this results from the fact that one of 

 the cells of the follicle usually acquires a very intimate relation 

 with the ovum, through a fine pseudopodial process, so that 

 at this point no membranes are laid down (Fig. 50). When the 



