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 
vitelline membrane; it always has a granular appearance, and if it 
were not for the fine vertical strie, 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 this (Plate 
XXIX. fig. 5), which I believe to be the thickened vitelline mem- 
brane of earlier stages. 
These pores are very obvious indeed, and impossible to be over- 
1 Quart. Journ. Mier. Sci, 1878, p. 405. 
