MATURATION OF GAMETES 167 



At one end of the forming egg, a follicle cell sends an enlarged pseudopodium- 

 like process inward to the surface of the egg. As a result of this enlarged 

 extension of the follicle cell to the ooplasmic surface, an enlarged pore-like 

 opening in the zona radiata is formed. This opening persists as the micropyle 

 after the egg leaves the ovary (fig. 93A). 



As the teleost egg is spawned, the chorionic layer hardens when it comes 

 in contact with the water. If fertilization occurs, the surface of the egg emits 

 a fluid and shrinks inward from the zona radiata. In this manner, a peri- 

 vitelline space is formed between the egg, and the zona is filled with a fluid. 

 The egg is thus free to revolve inside of the zona (Chap. 5). 



e) Vitelline Membrane (Zona Radiata) in Amphibia. In the am- 

 phibia, a vitelline membrane is formed probably by the surface ooplasm, 

 although there may be contributions by the follicle cells of the ovary (Noble, 

 '31, p. 281). This membrane separates from the egg at the time of fertiliza- 

 tion, forming a perivitelline space (fig. 94). The latter space is filled with 

 fluid. Later the vitelline membrane expands greatly to accommodate the de- 

 veloping embryo. A delicate surface layer or membrane forms the outer por- 

 tion of the ooplasm below the vitelhne membrane. In some amphibia the 

 vitelline membrane may have a radiate appearance. 



f) Zona Radiata (Zona Pellucida) of the Reptile Oocyte. In the 

 turtle group, the development of the zona radiata (pellucida) appears to be 

 the product of the follicle cells (Thing, '18). Filamentous prolongations of 

 the follicle ceUs extend to the surface ooplasm of the developing egg (fig. 95). 

 A homogeneous substance produced by the follicle cells then fills the spaces 

 between these prolongations. The filamentous extensions of the follicle cells in 

 this way produce a radiating system of canals passing through the homogeneous 

 substance; hence the name, zona radiata. Bhattacharya describes Golgi sub- 

 stance as passing from the follicle cells through the canals of the zona radiata 

 into the egg's ooplasm in the developing eggs of Testudo graeca and Uromastix 

 hardwicki. (See Brambell, '25, p. 147.) 



In contradistinction to the above interpretation, Retzius ('12) describes the 

 homogeneous substance which forms the zona radiata of the lizard, Lacerta 

 viridis, as originating from the ooplasm of the egg. 



g) Vitelline Membrane (Zona Radiata) of the Hen's Egg, The 

 vitelline membrane, as in the turtle groups, appears to form about the young 

 oocyte as a result of contributions from the surrounding follicle cells although 

 the superficial ooplasm of the oocyte may contribute some substance. This 

 occurs before the rapid deposition of yolk within the developing oocyte. It 

 is probable that the follicle cells send small pseudopodium-like strands of 

 cytoplasm through the numerous perforations of the very thin vitelline mem- 

 brane around the oocyte's surface into the superficial ooplasm in a similar 

 manner to that which occurs in reptiles. The vitelline membrane (zona radiata) 

 thus assumes a radiate appearance as it increases in thickness (figs. 47, 96). 



