GERMINAL MEMBRANE. 48 



membrane, the double outline of which he recognised, (plate 

 II, fig. I, d.) It is thus highly probable that the yelk of the 

 mammalian ovum is a cell. Even if, as Wagner intimates, the 

 vitelline membrane in other animals should sometimes be 

 formed only secondarily within the chorion, it would not 

 materially interfere with our purpose, since in that case the 

 chorion would be the cell-membrane. The ovum universally 

 possesses an external closed membrane (whether it be chorion 

 or vitelline membrane), which is structureless, and not gene- 

 rated from other elementary structures, and therefore is the 

 ovum always a cell. The yelk-cell encloses the vitelline sub- 

 stance as its cell-contents, and upon its internal surface lies 

 the germinal vesicle, or vesicle of Purkinje, (fig. 1, f.) 

 This, as is known, is a very transparent thin-walled vesicle, 

 containing a pellucid fluid, according to Wagner coagulable by 

 spirits of wine. It encloses almost universally (Wagner cites 

 but very few exceptions) upon the internal surface of its wall, 

 a corpuscle, called by the discoverer, R. Wagner, germinal spot, 

 or germinal disc, (fig. 1, g.) In mammalia it is generally flat. 

 In many instances several of these spots are present, their 

 number, however, is said by Wagner to bear proportion to the 

 age of the ovum, they being fewer and much more firmly 

 attached to the wall of the germinal vesicle in young ova. I have 

 frequently observed in osseous fishes (where they are often pre- 

 sent in such numbers as to prevent the fluid in the vesicle from 

 being seen) that when one of these corpuscles, after the bursting 

 of the germ-vesicle, passed through a narrow space, it first 

 became considerably elongated, and then drawn out in the 

 centre to a thin thread, which soon broke. The two ends 

 afterwards retracted, and thus two round globules were pro- 

 duced from one corpuscle, in a similar manner to what we may 

 observe in the drops of fat upon soup. They appear, therefore, 

 to be composed of a tenacious substance which is not misciblc 

 with water. Purkinje states that the germinal vesicle in birds 

 is firmly fixed to the vitelline membrane, but Baer and 

 Wagner describe it as lying in the centre of the yelk at first, 

 and rising to the surface at a subsequent period. 



The decision of the question, as to the precise signification 

 of the germinal vesicle, now becomes of great importance. Is 

 it a young cell generated within the yelk-ccll, or is it the 



