274 MR. F. E. BEDDARD ON THE [May 4, 



membrane shows signs of its subsequent reabsorption into the ovum, 

 the lower margin being rather less distinct than in the earlier stage. 

 At no time is the zona radiata so distinctly " membranous " as the 

 viteUine membrane ; it always has a granular appearance, and if it 

 were not for the fine vertical striae, which are presumably the expres- 

 sion of pores, would be regarded as merely a superficial layer of the 

 protoplasm appearing more deeply stained because of its firmer tex- 

 ture. If the vertical striation be really due to protoplasmic processes 

 traversing the zona radiata, this latter must be regarded as a distinct 

 metamorphosis of the superficial layer of the ovum ; otherwise, if it 

 really has retained its protoplasmic character, there would be no 

 necessity for special protoplasmic filaments to pass through its 

 substance : the whole layer would serve equally well as a conductor 

 of nutritive material. The disappearance of this membrane, which 

 commences with the commencing formation of yolk, is complete in 

 the later stages of the maturation of the ovum ; and the disappearance 

 is easier to understand on the assumption that the zona radiata is 

 only but little changed from its original protoplasmic condition, its 

 reabsorption being therefore a kind of solution. On the other hand, 

 the thinning of the vitelline membrane in the later stages of egg- 

 development is possibly purely mechanical, being due to the exten- 

 sion of an elastic membrane through the increasing bulk of the egg- 

 contents. 



The pores in the zona radiata are very generally believed to admit 

 processes of the follicular epithelium, and where no imperforate 

 membrane exists between the zona radiata and the follicular epithe- 

 lium as in Mammals, it has been actually demonstrated that such is 

 the case. 



In Elasmobranchs Balfour ' has described an enlargement of some 

 of the follicular cells as the ovum approaches maturity ; these are 

 doubtless concerned with the nutrition of the ovum, but they 

 cannot (?) come into actual contact with it because the vitelline 

 membrane divides them. It is a significant fact, however, that at 

 this period the vitelline membrane becomes extremely thin, so that 

 it would evidently favour osmosis. In this case the perforations of 

 the zona radiata may still be the expression of pores which contain 

 prolongations, not of the follicular cells, but of the egg-protoplasm, 

 which thus takes an active part in its own nutrition, as in the lower 

 forms, and is not merely passively fed by the follicular epithelium. 



This may well be the case with Lepidosiren, though I have 

 been unable actually to demonstrate the protoplasmic contents of the 

 pores in the zona. 



During this second stage of the egg-development, while the yolk 

 is in process of formation, and there is therefore a special need of 

 extra nutrition, not merely are these pores in the zona radiata, but 

 in the more highly refracting membrane outside of tliis (Plate 

 XXIX. fig. 5), which I beUeve to be the thickened vitelline mem- 

 brane of earlier stages. 



These pores are very obvious indeed, and impossible to be over- 

 ' Quart. Jcairn. Micr. Sci. 1878, p. 405. 



