62 



CHORDATA. 



chapter. In the ova with a large amount of food-yolk it assumes 

 an eccentric position very early. 



The homologies of the primary egg-membranes of Craniata 

 are still involved in some obscurity. There seem to be three 

 membranes, which may all coexist, and of which one or more 

 are almost always present. These membranes are 



(1) An outermost usually homogeneous non-perforated 

 membrane, which is by most authors regarded as a chorion, 

 but is probably a vitelline membrane by which name I shall 

 speak of it. 



(2) A radiately striated membrane (internal to the former 

 when the two coexist) which can be broken up into a series of 

 separate columns. These give to the membrane its radiate 

 striation, but it is probable that between the columns there are 

 pores sufficiently large to admit of the passage of protoplasmic 

 filaments. This membrane will be spoken of as the zona radiata. 

 It is a differentiation of the outermost layer of the yolk. 



(3) Within the zona radiata a third and delicate membrane 

 is occasionally found, especially when the ovum is approaching 

 maturity. 



In Elasmobranchii the first membrane to 

 be formed is the vitelline membrane, which 

 appears in some instances before the forma- 

 tion of the follicle a fact which appears to 

 shew that it is really formed as a differentia- 

 tion of the protoplasm of the egg. In most 

 Elasmobranchii this membrane attains a 

 very considerable development. A zona 

 radiata is generally (if not always) present 

 in Elasmobranchii, but arises at a later 

 period than the vitelline membrane (fig. 21, 

 Zri). The zona radiata always disappears 

 long before the ovum is ripe. The vitelline 

 membrane also gradually atrophies, though 

 it lasts much longer than the zona radiata. 

 When the egg is taken up by the oviduct all trace of both mem- 

 branes has vanished. In Reptilia precisely the same arrange- 

 ments of the membranes are found as in Elasmobranchii, except 

 that as a rule the zona radiata is relatively more important 



FIG. 21. SECTION 

 THROUGH A SMALL 

 PART OF THE SURFACE 

 OF AN OVUM OF AN IM- 

 MATURE FEMALE OF 

 SCYLLIUM CANICULA. 



fe. follicular epithe- 

 lium, vt. vitelline 

 membrane. Zn. zona 

 radiata. yk. yolk with 

 protoplasmic network. 



