[15] EMBRYOGRAPHY OF OSSEOUS FISHES. 469 



let, lias given tlie followiug account of the changes undergone by ihe 

 germinative vesicle: 



" The germinal vesicle is embedded in the germ disk, and is so large 

 that it may be seen with the naked eye; it is without a wall, and con- 

 sists of a glairy substance, which hardens in spirits, and is only sepa- 

 rated from the surrounding plasma of the germ by a denser layer of 

 protoleucite [germinal protoplasm]. 



" During the first hours after oviposition one can no louger distin- 

 guish the germinal vesicle; and in its place a number of small isl- 

 ands maybe observed, consisting of more transparent matter, which 

 are scattered through the germinal mass, and which in their structure 

 are quite similar to the germinal vesicle. The identity of the sub- 

 stance of these islands with that of the germinal vesicle indicates 

 that the latter, even before impregnation, breaks up into a number of 

 parts ; a phenomenon which is analogous to what occurs in other ani- 

 mals, as in echinoderms, for example, as described by several investi- 

 gators (O. Hertwig and Fol). 



" Impregnation is indicated by the appearance of a clearer discoidal 

 mass at the upper pole of the egg, and consists of a transparent, almost 

 homogeneous substance, which corresj)onds to the veil-like body of the 

 amphibian ovum described by Hertwig. We may retain the name 

 proposed by Hertwig for this structure. On the surface of the veil- 

 like body a vast number of spermatozoa may be noted, with their 

 heads directed to the outside and their tails in the ojjposite direction. 

 The veil like body lies so closely against the surfoce of the egg that it 

 is difficult to make out a separation between them ; it attains its great- 

 est thickness at the upper jjole of the egg, and grows downwards in the 

 form of a strand into the germinal mass ; towards its margin the veil- 

 like body gradually thins out. The surface of the germ at the time of 

 impregnation appears to be very strongly i^igmented. At the upper 

 pole of the egg this pigmentation is most marked. The pigmented 

 mass, which appears in that position as an elevation, depends inwards 

 into the germ and forms a band, which, from its analogy to that de- 

 scribed in the ova of amphibia by O. Hertwig and Bambeke, may also 

 be called the pigmented tract (Pigmentsfrasse). It is very possible that 

 this indicates the pathway by which the spermatozoa penetrate the 



" The entrance of the spermatozoa I could not observe. For the ob- 

 servation of this phenomenon the eggs of the sterlet are not well 

 a<lapted. In the earliest stages observed by me I found a clear spot 

 at the lower end of the pigmented tract which was evidently nothing 

 more than a portion of the future segmentation nucleus, and therefore 

 the male pronucleus. This body was without a wall, and consisted of 

 a finely granular, transparent substance, and was covered above by pig- 

 ment granules. The formation of the female pronucleus {Eikernes, O. 

 Hertwig) occurs at the expense of the islands already alluded to, one 



