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Show 1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. 507 division of the ovum itself into two zones by a definite membrane would seem to be an absurdity; in the second place, no membrane was evident in preparations where the ovum was broken. It might be expected that when the ovum was broken in cutting, the membrane, being presumably of a different hardness to the egg-protoplasm, would project from the cut surface; in no instance, however, did the broken surface show any indication anywhere of a membrane. The line of division between the two yolk-zones presented the appearance in my preparations of an absolute break ; the protoplasm was perfectly transparent, and, being unaffected by the staining-reagent, was invisible. I did not notice this differentiation of the yolk in all the large ova visible in m y sections. In some ova, which were full of yolk, and of equal size with those just referred to, there was no trace of any such specialization into a peripheral and central zone; in these cases the yolk was uniform throughout. Such ova were to be found not only in the same ovary, but in the same section with the ova which displayed a differentiation of the yolk. This circumstance renders it improbable that the effect of reagents has caused the yolk to acquire a uniform appearance. A comparison of the two kinds of ova has led me to the conclusion that the ova in which there is a specialization of the yolk are nearly mature, while those in which the yolk is uniform are degenerating ova. Another matter relates to the structure of egg-membranes and their homologies, where I have to make a correction. In m y former paper I have referred to the presence in comparatively young ova of a vertically striate membrane lying within the vitelline membrane (loc. cit. p. 273, pi. xxviii. fig. 1, z.r; pi. xxix. fig. 2, z.r). This, it now appears to me, is not the equivalent of the inner of the two membranes which surround the Teleostean ovum1. The early disappearance of this membrane and its general structure (granular and with no distinct line of separation from the subjacent egg-protoplasm) were against such an interpretation ; I now identify it with more confidence with a specialized layer of the egg-protoplasm described by Brock in Alburnus lucidus, Salmo fario, and Perca fluviatilis, and by Owsiannikow in Acerina vulgaris. Brock has figured this layer (the " Zonoidschicht" of His, the " helle Rand-schicht" of Gegenbaur) in Alburnus lucidus (Morph. Jahrb. Band iv. pi. xxviii. fig. \2,fg.), where it is more complicated than in Lepidosiren and consists of two layers-an inner homogeneous and an outer vertically striate layer. CONTENTS OF THE OVARY OF PROTOPTERUS. The following is a detailed account of the structure and development of certain bodies in the ovary which have already been partly described in m y former paper ; they are nearly as numerous as the ordinary ova. 1 Cf.J.T. Cunningham, " On the Mode of Attachment of tbe Ovum of Osmerus perlanus," P. Z. S. 1886, pt. iii. p. 292, pi. xxx. fig. 4, z.r.i, and other memoirs. |
