1886.] OVUM OF LEPIDOSIREN. 2S3 



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, vyith 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 such a supposition I may adduce the followingarguments: — 

 (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 ej)ithelium 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. Gotte, 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 

 zusammenhangen." It is only later that a membrane surrounding 

 the ovum and separating it from the follicular cells appears. 



Similarly Iwakawa's observations upon Triton ' show that in the 

 young stages the folhcular cells are in contact with the body of the 

 ovum. In more mature ova a membrane conies to svuround the 

 ' Quart. Joiini. Mior. Sci. vol. sxii. )). 270, 



19* 



