1886. ] STRUCTURE OF THE OVUM IN THE DIPNOI. 523 
oue of the cells, which compose the central mass of the follicle, 
acquiring a predominance in size over its neighbours or being differ- 
entiated in any other way. 
The fact of there being two kinds of ova with a different mode of 
development is not new to the Vertebrata. In my former paper I 
have compared the follicle and its contents in Protopterus to the 
‘*egg-nest” of Elasmobranchs, the points of difference being perhaps 
on the whole greater than the points of resemblance. But, since 
the formation of “ egg-nests” is so general among the Vertebrata, 
it seems to me that there is probably some genetic connection 
between these structures and the “egg-nest”’ of Protopterus. It 
has been shown that in Mammals, Elasmobranchs, and Reptiles 
the permanent ova are formed in two ways:—either (1) by the 
direct development of one of the primitive ova, which surrounds itself 
with a follicular layer derived from the ordinary undifferentiated 
germinal cells; (2) a number of primitive ova coalesce together to form 
anest ; their nuclei multiply, and some atrophy, serving as pabulum 
for a limited number which subsequently separate off, accompanied 
by some of the undifferentiated germinal cells, to form as many ova. 
These two modes of development are not regarded by Balfour as 
morphologically very different ; the latter mode of development has 
been brought about to secure the adequate nourishment of a certain 
number of cells which form the permanent ova. 
Balfour’s* observations certainly do not show any morphological 
difference between the ova produced in these two different ways; in 
both cases the ovum is the equivalent of a single cell; but the 
physiological difference is considerable. 
I have referred above (p. 512) to the presence of two kinds of cells 
among the central cells, distinguishable by the characters of their 
nuclei; in some the nucleus was rather larger and more oval in form 
than in others, where it was smaller and rounder, and, in fact, exactly 
like the nucleus of a follicular cell. It is possible that the cells with 
larger nuclei correspond to primitive ova and the smaller to the 
ordinary germinal cells ; in this case the resemblance of the central 
mass of cells to the egg-nest of the Elasmobranch will be dimin- 
ished ; against this supposition is the fact that there are nuclei of 
intermediate size, but these may have been produced by a recent 
division of some of the other nuclei. 
Judging from analogy, however, it is probable that some of the 
cells of the germinal epithelium are specialized into primitive ova from 
undifferentiated germinal cells, although in patches of germinal epi- 
thelium covering the ovary I failed to detect any such specialization 
in the nuclei. 
The important facts to be borne in mind in comparing the ege- 
nest of the Elasmobranchwith that of the Dipnoi® appear to me to be 
the early formation of the complicated follicular layers in the latter and 
the early commencement of yolk-secretion. The germinal cells being 
1 Balfour, loc. cit. 
? It is important to remember that the formation of ‘‘nests” is not confined 
in Elasmobranchs to the embryonic period (see Balfour, loc. cit, p. 415). 
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