FECUNDATION IN FISHES. 



601 



421 



ensure intromission. The superadded claspers in the male Plagio- 

 stomes lend more effectual aid in the act of internal impregnation, 

 for in those species that are oviparous the ova are impregnated and 

 covered by a nidamental coat, or ' shell,' prior to exclusion. 



In Osseous Fishes, where exclusion usually precedes impregna- 

 tion, the first change observed in the 

 ovum after entering the water is its im- 

 bibition, causing a separation of the outer 

 tunic from that of the yolk, fig. 421, a. 

 Dr. Ransom connects the phenomenon 

 with the passage of the spermatozoa 

 through the micropyle, which he ob- 

 served in his experiments on the ova 

 of the Stickleback. 1 In these ova, 

 about three minutes after impregnation, 

 the funnel of the micropyle, which had 

 descended into a depression on the 

 upper surface of the germinal part, fig. 

 421, b, began to be withdrawn by the 

 recession of the external membrane from 

 the surface of the yolk and the forma- 

 tion of the intervening clear space. 

 About ten minutes after impregnation 

 the clear respiratory space is more 

 marked : the germinal layer, with a few 

 large oil-globules, is distinguishable by 

 its opacity from the clearer part of 

 the yolk, ib. a. In the Perch it presents a greyish, in the Pike 

 a yellowish, tint. The germinal vesicle, which had previously 

 become filled and obscured by granules and granular corpuscles, 

 breaks up to form, or contribute to form, the germinal layer, 

 which now becomes more circumscribed and distinct : the process 

 of segmentation, which follows that of impregnation, is limited 

 to the germinal portion of the yolk, with which it is co-extensive. 

 In the Perch the ova assume a greenish tint shortly after impreg- 

 nation. There is reason to suppose that impregnation of the 

 eggs of both Sharks and Rays takes place in the ovarium or the 

 contiguous part of the oviduct, for they become enveloped in the 

 dense albuminous secretion of the nidamental glands after having 

 passed that part, which covering would prevent the subsequent 

 influence of the spermatozoa. 



§ 115. Develojyement of Fishes. — The germinal layer consists of 



1 Cited in cccviii. p. 98. 



Ovum of the Gasterosteus at the timo 

 of impregnation. 



