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 1t 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 1s 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 6th they are 
invaginated and the lenses are formed so as to le 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 off 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. 
