2l6 THE DEVELOPMENT OF ELASMOBRANCH FISHES. 



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( i) The thickness of its upper surface. (2) The extension of 

 its edge over the yolk. (3) Its position external to the yolk. 



In favour of the view that the contents will be left behind 

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

 ing features of my sections : 



(i) The rupture of the membrane of the germinal vesicle on 

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

 pletely 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 suggest themselves 1 . 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 aggre- 

 gated together in a hollow of the yolk After this a hole appears in the 



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 the 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 the 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 demonstrating the extrusion of the germinal vesicle 

 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 membrane 

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

 vesicle, contents and all. 



1 Archiv fiir Micr, Anat. Vol. VIII. p. i. ' 



