DEVELOPMENT OF ELASMOBRANCH FISHES. 7 



out through the membrane. This, which so far was firmly stretched, 

 next collapses to a somewhat egg-like sac, whose wall is thickened 

 and in places folded. 



" The inner mass which up to this time has remaiaed compact 

 now breaks up into separate highly refi-active bodies, of spherical 

 or angular form and of very different sizes ; between them, here and 



there, are scattered drops of a fluid fat I am very much inclined 



to regard the solid bodies in question as fat or as that peculiar 

 modification of albuminoid bodies which we recognise as the certain 

 forerunner of the formation of fat in so many pathologically altered 

 tissues; and therefore to refer the disappearance of the germinal 

 vesicle to a fatty degeneration. On one occasion I believe that I 

 observed an opening in the membrane at this stage ; if this is a 

 normal condition it would be possible to believe that its solid con- 

 tents passed out and were taken up in the surrounding plasma. 

 What becomes of the membrane I am unable to say ; in any case 

 the germinal vesicle has vanished to the very last trace before 

 impregnation occurs." 



Kleinenberg clearly finds that the germinal vesicle disappears 

 completely before the appearance of the Richtungskorper, in 

 which he states a pseudocell or yolk-sphere is usually found. 



The connection between the Richtungskorper and the germi- 

 nal vesicle is not a result of strict observation, and there can 

 be no question that the evidence in the case of invertebrates 

 tends to prove that the germinal vesicle in no case disappears 

 owing to its extrusion from the egg, but that if part of it is 

 extruded from the egg as Richtungskorper this occurs when its 

 constituents can no longer be distinguished from the remainder 

 of the yolk. This is clearly the case in Hydra, where, as stated 

 above, one of the pseudocells or yolk-spheres is usually found 

 imbedded in the Richtungskorper. 



My observations on the Skate tend to shew that, in its case, 

 the membrane of the germinal vesicle is extruded from the egg^ 

 though they do not certainly prove this. That conclusion is 

 however supported by the observations of Schenk\ He found 

 in the impregnated, but not yet segmented, germinal disc a 

 cavity which, as he suggests, might well have been occupied 

 by the germinal vesicle. It is not unreasonable to suppose that 

 the membrane, being composed of formed matter and able only 

 to take a passive share in vital functions, could, without thereby 

 influencing the constitution of the ovum, be ejected. 



If we suppose, and this is not contradicted by observation, 



1 Die Eier von Eaja qua<Irimaculata, Sitz. der k. Akad. Wien, Bd. lxviii. 



