1886. | OVUM OF LEPIDOSIREN. 283 
means of the proliferating follicular cells. I do not mean to assert 
that these ova do not also form yolk endogenously, as I have observed 
in other cases (Plate XXVIII. fig. 2); but the similarity between the 
contents of the follicular cells and their derivatives which migrate 
into the interior of the ovum, on the one hand, with the yolk-spherules 
of the ovum, on the other hand, coupled with the fact of the subse- 
quent disintegration and disappearance of the immigrated cells, seems 
to me to necessitate the conclusion that at least part of the yolk is 
so formed. 
Unfortunately I am quite unable to record any observations as to 
the earlier stages of these ova; they may be a stage intercalated 
between Stages 3 and 4, as already suggested. In favour of this 
supposition are the following facts :—(1) that for the most part these 
ova are intermediate in size between the presumed earlier and later 
stages ; (2) that there is no proof of their independent origin ; (3) the 
unlikelihood (?) of there being two kinds of ova with a different 
process of growth. 
Against suchasupposition I may adduce the following arguments:— 
(1) the absence of any transitional stages between these ova and the 
presumed younger stages; (2) the disappearance of the vitelline 
membrane during this stage and its subsequent reappearance, or at 
least the appearance of a similar membrane. 
Whatever may be the origin of these ova, they come ultimately to 
resemble in every particular the ordinary mature ova. I have 
fortunately succeeded in obtaining sections of an ovum considerably 
larger than that just described, which presented the following 
characters :—The follicular epithelium was considerably reduced in 
importance, as was also the accompanying network of blood-capil- 
laries. Between the follicular epithelium and the ovum was a 
distinct membrane excessively thin ; there were hardly any traces 
left of the immigrating follicular cells present in such great numbers 
in the earlier stages; I noted perhaps one or two in as many 
sections. 
It is interesting to observe that if these ova, characterized by 
the formation of the yolk from the follicular cells, are really different 
from the other ova, they are in certain respects more Amphibian 
like than the other ova; not in the immigration of follicular cells, 
but in the late appearance of a single thin membrane shutting off 
the ovum from the follicular epithelium with which it was previously 
in contact. Gdtte, in his ‘ Entwickelungsgeschichte der Unke,’ makes 
the following statement (p. 16):—‘* Ferner kann man bei der 
Anwendung des Wassers nachweisen dass der Follikelinhalt noch 
unmittelbar die Zellen beriihrt, dass aber die Grenze zwischen beiden 
Theilen eine sehr scharfe ist und sie durchaus nicht continuirlich 
zusammenhiingen.” It is only later that a membrane surrounding 
the ovum and separating it from the follicular cells appears. 
Similarly Lwakawa’s observations upon Triton’ show that in the 
young stages the follicular cells are in contact with the body of the 
ovum. In more mature ova a membrane comes to surround the 
* Quart. Journ. Mier. Sei. vol, xxii. p. 270. 
19* 
