194 
J. D. F. GILCHRIST. 
both were enveloped in a common follicle was not sufficient in 
itself to disprove this, but, as no nucleus could be found in the 
larger mass, it must be concluded that we are dealing* here 
with one ovum. More advanced ova, completely transformed 
into yolk granules and yolk nuclei, possessed a large nucleus 
and nucleolus, located now in the centre. 
The origin and nature of the yolk nuclei in general is still 
an obscure question. It has been suggested that they arise 
independently in the cytoplasm, that they are derived from 
the nucleus, and that they are derived from the nucleolus. 
The evidence in this case seems to be in favour of the last 
suggestion, in view of the appearances in the nucleolus noted 
above. The nuclei did not appear to originate from a single 
large yolk nucleus as is the case in other instances of such 
structures. The further study of the change in the ovarian 
egg seems to be worthy of attention. 
The Fertilised Ovum. 
Two or three specimens only of the unsegmented ovum, 
enclosed in its clear capsule and presumably fertilised, were 
procured. Such eggs (PI. 13, fig. 2) were quite spherical, in 
contrast to later stages. In sections, among the numerous 
eosin-stained yolk granules, were seen some small bodies, 
stained, though not conspicuously, with haematoxylin, pre- 
sumably the yolk nuclei. 
The eggs of Cephalodiscus have been described as oval 
and of a varying diameter. These are probably late stages 
of the ovum, in which the embryo is fairly advanced, and the 
egg proper may not vary much in diameter, though sufficient 
material at this stage was not available to give any certainty 
on this point. 
Segmentation . 
The first division of the ovum, from the beginning of the 
constriction to the complete separation of the blastomeres, 
was observed. More examples of this stage were found than 
