356 CHORDATA. 
The body is now distinctly elongated and compressed slightly from 
side to side. 
This stage may (except for the closure of the neuroproe and preco- 
cious formation of the brain) be compared to the chordula larva of the 
Fig. 254.—THE STRUCTURE OF FRoc’s EMBRYO AND TADPOLE. 
Head. 
bse. 
tA) 
Crp 
ry B 
Tort DD 
¢ Stomodzum. 
Anus etre 2 
oo oe 
Second Gill. 
First Gill. | Ear. 
Eye. 
Anus. 
Notochord. Pronephric Duct. 
Neurenteric 
Canal. 
nus. 
Yolk-cells. Liver. Heart. 
A Longitudinal Median Section through an Embryo of Frog X 20 (after 
MarsHatt). Note the dorsal nerve-tube swollen into a brain anteriorly, the noto- 
chord below it, and the mass of yolk-cells on the ventral wall of the archenteron. 
RB, Side (right) View of just-hatched Tadpole of Frog x 6 (ad at.). C, Median 
Longitudinal Section through a newly-hatched Frog’s Tadpole (chiefly after 
MARSHALL). 
Alriozoa. As in the case of the gastrula, the chief difference is in the 
presence of a large mass of yolk containing hypoblast on the ventral 
wall of the archenteron. 
