MATURATION OF GAMETES 165 



represents the thin surface layer of ooplasm is present. However, it does not 

 separate from the periphery of the egg at fertilization. During its early devel- 

 opment, the embryo remains within this chorionic shell. The chorion thus 

 represents the primary embryonic membrane of this species. 



b) Egg Membranes of Amphio.xus. Two surface membranes are formed 

 and eventually separate from the egg of Amphio.xus. The outer vitelline 

 membrane is elaborated on the surface of the egg and remains in contact 

 with this surface until about the time of the first maturation division. It then 

 begins to separate from the egg's surface. (See Chap. 5.) After the sperm 

 enters and the second maturation division occurs, a second, rather thick, 

 vitelline membrane also separates from the egg. The first and second vitelline 

 membranes then fuse together and become greatly expanded to form the pri- 

 mary embryonic membrane. (See Chap. 5.) A thin ooplasmic membrane 

 remains at the egg's surface. 



c) Vitelline Membrane and Zona Radiata of Elasmobranch Fishes. 

 In the egg of the shark, Scyllium canicula, two egg membranes are formed, 

 an outer and an inner membrane. The outer membrane is a homogeneous 

 vitelline membrane, while the membrane which comes to lie beneath this 

 outer membrane has a radiate appearance and hence may be called a zona 

 radiata. This latter membrane soon loses its radiate appearance and becomes 

 a thin membrane along the inner aspect of the vitelline membrane (fig. 92A, 

 B). In the mature egg both of these membranes form a thin, composite, 

 vitelline membrane (fig. 92C, D). At about the time of fertilization the latter 

 membrane separates from the egg's surface; a perivitelline space then lies 

 between these structures and the surface ooplasm of the egg. 



d) Zona Radiata of Teleost Fishes. The surface ooplasm in teleost 

 fishes gives origin to a membrane which in many cases has a radiate appearance. 

 In some species this membrane appears to be composed of two layers. This 

 radiate membrane which forms at the surface of the egg of teleost fishes 

 appears to be the product of the ooplasm, and, therefore, should be regarded 

 as a true vitelline membrane. In the perch a true chorion also is formed 

 as a gelatinous or filamentous layer produced external to the radiate mem- 

 brane by the follicle cells (fig. 93B). In Fundulus heteroclitus there are 

 apparently three distinct parts to the membrane which surrounds the ooplasm 

 of the egg: 



( 1 ) a zona radiata, 



(2) a thin structureless membrane external to the zona, and, 



(3) the filamentous layer whose filaments are joined to the thin membrane 

 around the zona (fig. 93C). 



These three layers are probably derived from the ooplasm of the egg 

 (Eigenmann, '90). Consequently, the filamentous chorion or gelatinous layer, 

 if derived from the egg itself, is not a true chorion in this particular egg. 



