4 EIPE OVAEIAN OVUM. 



and absorbed when the membrane is pushed out, are the follow- 

 ing features of my sections : 



(1) The rupture of the membrane of the germinal vesicle 

 on its lower surface. (2) The position of the contents almost 

 completely below the membrane of the vesicle and surrounded 

 by yolk. 



In connection with this subject, Oellacher's valuable observa- 

 tions upon the behaviour of the germinal vesicle in Osseous 

 Fishes and in Birds at once suoffj^est themselves \ Oellacher 

 sums up his results upon the behaviour of the germinal vesicle 

 in Osseous Fishes in the following way (p. 12) : 



" The germinal vesicle of the Trout's egg, at a period when the 

 egg is very nearly ripe, lies near the surface of the germinal 



disc which is aggregated together in a hollow of the yolk 



After this a hole appears in tlie membrane of the germinal vesicle, 

 which opens into the space between the egg-membrane and the 

 germinal disc. The hole widens more and more, and tlie membrane 

 frees itself little by little from the contents of the germinal vesicle, 

 which remain behind in the form of a ball on the floor of the cavity 

 formed in this way. The cavity becomes flatter and flatter and the 

 contents are pushed up further and further from tlie germinal disc. 

 "When the hollow, in which lie the contents of the original germinal 

 vesicle, completely vanishes, the covering membrane becomes inverted 



and the membrane is spread out on the convex surface of the 



germinal disc as a circular, investing structure. It is clear that by 

 the removal of the membrane the contents of the germinal vesicle 

 become lost." 



These very definite statements of Oellacher tell strongly 

 against my interpretation of the appearance presented by the 

 germinal vesicle of the ripe Skate's egg. Oellacher's account is 

 so precise, and his drawings so fully bear out his interpretations, 

 that it is very difficult to see where any error can have crept in. 



On the other hand, with the exception of those which 

 Oellacher has made, there cannot be said to be any satisfactory 

 observations demonstratins^ the extrusion of the sferminal vesi- 

 cle from the ovum. Oellacher has observed this definitely for 

 the Trout, but his observations upon the same point in the 

 Bird would quite as well bear the interpretation that the mem- 

 brane alone became pushed out, as that this occurred to the 

 germiual vesicle, contents and all. 



1 Arcliiv fur Micr. Anat. Vol. viii. p. 1. 



