2l8 DEVELOPMENT OF ELASMOBRANCH FISHES. 



extrusion of the vesicle in the way described by Oellacher would 

 have escaped his notice. 



Passing from Vertebrates to Invertebrates, we find that 

 almost every careful investigator has observed the disappear- 

 ance, apparent or otherwise, of the germinal vesicle, but that 

 very few have watched with care the steps of the process. 



The so-called Richtungskorper has been supposed to be the 

 extruded remnant of the germinal vesicle. This view has been 

 especially adopted and supported by Oellacher (loc. cit\ and 

 Flemming 1 . 



The latter author regards the constant presence of this body, 

 and the facility with which it can be stained, as proofs of its 

 connection with the germinal vesicle, which has, however, accord- 

 ing to his observations, disappeared before the appearance of the 

 Richtungskorper. 



Kleinenberg 2 , to whom we are indebted for the most precise 

 observations we possess on the disappearance of the germinal 

 vesicle, gives the following account of it, pp. 41 and 42. 



"We left the germinal vesicle as a vesicle with a distinct doubly con- 

 toured membrane, and equally distributed granular contents, in which the 



germinal spot had appeared The germinal vesicle reaches o - o6mm. in 



diameter, and at the same time its contents undergo a separation. The 

 greater part withdraws itself from the membrane and collects as a dense 

 mass around the germinal spot, while closely adjoining the membrane there 

 remains only a very thin but unbroken lining of the plasmoid material. The 

 intermediate space is filled with a clear fluid, but the layer which lines the 

 membrane retains its connection with the mass around the germinal vesicle 

 by means of numerous fine threads which traverse the space filled with fluid. 



At about the time when the formation of the pseudocells in the egg is 



completed the germinal spot undergoes a retrogressive metamorphosis, it 

 loses its circular outline and it now appears as if coagulated ; then it breaks 

 up into small fragments, and I am fairly confident that these become 



dissolved. The germinal vesicle becomes, on the egg assuming a 



spherical form, drawn into an eccentric position towards the pole of the egg 

 directed outwards, where it lies close to the surface and only covered by a 

 very thin layer of plasma. In this situation its degeneration now begins, 

 and ends in its complete disappearance. The granular contents become 

 more and more fluid ; at the same time part of them pass 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. 



1 " Studien in der Entwicklungsgeschichte der Najaden," Sitz. d. k. Akad. /r/V//, 

 Bd. I. XXI. 1875. - Hydra. Leipzig, 1X7:. 



