1886.] STRUCTURE OF THE OVUM IN THE DIPNOI. riO? 



division of the ovum itself into two zones by a definite membrane 

 would seem to be an absurdity ; in the second place, no meaibrane 

 was evident in preparations where the ovum was broken. It mii;!;ht be 

 expected that when tlie 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 my 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 dis- 

 played 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 my former paper I have referred to the presence in compara- 

 tively 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 ovum \ 

 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 Aliurnus lucidus, Salmo fnrio, and Perca 

 Jiuviatilis, and by Owsiannikow in Acerina vulgaris. Brock has 

 figured this layer (the " Zonoidschicht " of His, the "helle Rand- 

 schicht" of Gegenbaur) in A/burnus lucidus (Morph. Jahrb. Band iv. 

 pi. xxviii. fig. \2,f.g.), where it is more complicated than in Lepido- 

 siren 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 develop- 

 ment of certain bodies in the ovary which have already been partly 

 described in my former paper ; they are nearly as numerous as the 

 ordinary ova. 



' Cf. 3. T. Cunningham, " On the Mode of Attachment of the Ovum of Osmerus 

 perlanus," P. Z. S. 1S8(3, pt. iii. p. 292, pi. xxx. fig. 4, z.r.i, and other memoirs. 



