| OCR Text |
Show 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 tbe 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 subsequent disintegration and disappearance of the immigrated cells, seems to m e 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 epithelium was considerably reduced in importance, as was also the accompanying network of blood-capillaries. 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. 1 6 ) : - " Ferner kann man bei der Anwenduug 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 1 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 1 Quart. Journ. Micr. Sci. vol. xxii. p. 270. 19* |
