50 



CHORD AT A. 



(1) An outermost usually homogeneous non-perforated mem- 

 brane, 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 mem- 

 brane 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 formation 

 of the follicle a fact which appears to shew 

 that it is really formed as a differentiation of 

 the protoplasm of the egg. In most Elasmo- 

 branchii this membrane attains a very con- 

 siderable development. A zona radiata is 

 generally (if not always) present in Elasmo- 

 branchii, but arises at a later period than the 

 vitelline membrane (fig. 21 Zn}. 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 arrangements of the membranes 

 are found as in Elasmobranchii, except that as a rule the zona radiata 

 is relatively more important. The vitelline membrane is thin except 

 in the Crocodilia. The third innermost membrane is found according 

 to Eimer in many Reptilia. In birds both vitelline membrane 

 and zona radiata are present, but the latter atrophies early, leaving 

 the former as the sole membrane when the egg is ripe. 



In osseous fish the vitelline membrane is usually either absent 

 or may perhaps in some instances, e.g. the Perch, be imperfectly 

 represented. In the ripe ovum of the Herring there is a distinctly 

 developed membrane external to the zona radiata which is probably 

 the vitelline membrane. The zona radiata attains a very great 

 development, and is generally provided with knobs of various shapes 

 on its outer surface. A delicate membrane internal to this my third 

 membrane has often been described, but there is still some doubt 

 about its existence. In some cases an external less granular layer of 

 the ovum itself has been described as a special membrane. In the 

 Perch a peculiar mucous capsule, penetrated by irregular branched 



\yk 



FIG. 21. SECTION 

 THROUGH A SMALL PART 

 OF THE SURFACE OF AN 

 OST.M OF AN IMMATURE 



FEMALE OF ScYLLIUJl 



CANICULA. 



fe. follicnlar epithe- 

 lium, vt. vitelline 

 membrane. Zn. zona 

 radiata. ?/A\ yolk with 

 protoplasmic network. 



