No. I.] 



GAR-PIKE AND STURGEON. 



39 



ence. The segmentation cavity has acquired a pronounced 

 deepening in the region of tlie end of coelenteron, but its 

 anterior limits, as before, remain difficult to be determined ; 

 its roof is now greatly reduced in thickness, in a large part 

 of its extent a single-celled layer ; it is densely pigmented and 

 its cell boundaries may be distinguished only with the greatest 

 difficulty. The immediate rim of the blastopore is thickened, 

 especially in the dorsal lip : here the thickened rim, by being 

 pushed against the yolk cells, has acquired a notch immediately 

 in front of it, which will later be discussed as the homologue of 

 Kupffer's vesicle. Pigment again occurs in all invaginated 



regions. 



Contrasted with Lepidosteus, this stage of the Sturgeon will 

 at once be seen to present differences which appear inter- 

 pretable as of specialized character ; thus we may contrast : 



Gastrula. 

 Segmentation cavity. 



Roof of cavity. 



Lepidosteus. 

 Two-layered. 



Simple. 



Thick, little pig- 

 mented. 



Acipenser. 

 Three-layered. 



Broadly dilated near end of coe- 

 lenteron, its anterior limit al- 

 most indiscernible. 



Thin, single-celled for a large ex- 

 tent of its surface, heavily pig- 

 mented. 



Walls of coelenteron. Unpigmented 

 Rim of blastopore. 



Thin, somewhat ta- 

 pering distally. 



Pigmented. 



Thick, blunted, thickest at dorsal 

 lip, where an anterior notch 

 (Kupffer's vesicle) is present. 



A late gastrula, PI. Ill, Fig. 50, shows the continued reduc- 

 tion of the blastopore, and the appearance in surface view of the 

 embryo's axis. The region surrounding the blastopore appears 

 slightly conical in its surface curvature, the opposite end of the 

 ^^g flattened and finely pigmented. The light-colored outer layer 

 and the blackened yolk plug of Sturgeon present a well known 

 contrast to the pigment conditions of the amphibian gastrula. 

 The embryo is in this stage but faintly discernible ; it appears as 



