-13- 



(v,), and the circulation of the blood through the branchial 

 arches can be seen easily under a low power. The thyroid, 

 being rather more opaque than the rest of the animal, shov;s very 

 plainly by contrast. In the figure the outlines of the pharynx 

 and the positions of the arohes are sho\7n by the broad dotted 

 lines. The characteristic hood-like projection over the raoiath 

 is also shovm. With the exception of the eye (e.), no attempt 

 has been made to represent any of the other organs of the larva. 



A ventral view of the larva of this age shows the thy- 

 roid as a clearly defined elongated body, lying against the ven- 

 tral wall of the pharynx. 



By this time great advances have been made in the de- 

 velopment of the thyroid and of the ciliated grooves. Fig. 5 b 

 is a transverse section a short distance anterior to the thyroid. 

 It shows the spinal cord (sp.c), notoohord (ch,), body wall 

 (b.w. ), etc. in outline, while the cell structure of the pharynx 

 (phar, ) is shov/n. This section, on the right side, passes through 

 the ciliated groove at the point (1) where its anteriorly-dlrectf' 

 ed, ventral branch (v.c*g., on the left) turns dorsalward to run 

 posteriorly as the dorsal groove (d.cg., on the left). On the 

 l-^ft of the section the two branches of the groove are distinct 

 as a dorsal (d.cg.) and ventral (v. e.g.) groove. These grooves 

 are lined with cells which are much higher, and more colamnar 

 than the cells of the rest of the pharyngeal wall, so that they 

 are easily follo\7ed„ by sections, throughout their course. 



