204 



axis are laid down, and the main divisions of the brain are 

 visible. The optic vesicles are formed but are not yet 

 invaginated, and no mesoblastic somites are as yet discern- 

 ible. The tail projects as a slight swelling into the 

 blastoporic area. Underneath the tail a small vesicle has 

 appeared, which increases in size with the waning of the 

 blastopore. This is Kupffer's vesicle (fig. 32), a structure 

 characteristic of the Teleostean embryo. It lies immedi- 

 ately beneath the posterior extremity of the notochord, 

 and in close relation to a slight depression under the 

 thickened blastodermic rim beneath the tail protuberance 

 — the " prostoma." Its cavity is bounded by a regular 

 layer of hypoblastic cells, or its dorsal wall is so formed, 

 and its ventral wall is formed by the yolk. In some 

 Teleosts it communicates with the exterior by a narrow 

 opening. It is the invagination cavity of the (masked) 

 Teleostean gastrula, and represents the archenteron. But 

 from its large size and persistence it is probably a func- 

 tional larval organ, and Sumner* has suggested that it 

 subserves the nutrition of the embryo by aiding in the 

 absorption of the yolk. 



On the 5th day constriction of the optic vesicles from 

 the mid-brain is complete, and on the Oth they are 

 invaginated and the lenses are formed so as to lie in the 

 openings of the optic cups. About 18 pairs of somites are 

 now present. The trunk has elongated considerably and 

 the head and tail are now constricted olf from the yolk 

 mass ; 8 days after fertilization the auditory vesicles are 

 present, the heart is formed and is beating, and the 

 vitelline circulation is being laid down. The tail has 

 increased considerably in length and about 30 somites 

 are present. On the 11th day rudiments of the pectoral 

 fin folds are present. 



* Mem. New York Acad., vol. ii, 1900, pp. 47-83. 



