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Show 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 striae, which are presumably the expression of pores, would be regarded as merely a superficial layer of the protoplasm appearing more deeply stained because of its firmer texture. 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 extension 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 epithelium, 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 X X I X . fig. 5 ) , which I believe to be the thickened vitelline m e m brane of earlier stages. These pores are very obvious indeed, and impossible to be over- 1 Quart. Journ. Micr. Sci. 1878, p. 405. |
