FROM TWELVE TO THIRTY-SIX SOMITES LATE 
small round dorsal, and long fissure-like ventral cleft at about 
the 40s stage (Fig. 102). These close during the fifth day. 
The significance of the separate dorsal and ventral divisions of the 
visceral clefts is an interesting question. It is probable that the dorsal 
division had a special function, as they have a special connection with 
the branchial sense organs. 
Fic. 102. — Reconstruction of the pharyngeal organs of the chick 
at the end of the fourth day of incubation. (After Kastsch- 
enko.) 
a.a. 3, a. a. 4, a. a. 6, Third, fourth, and sixth aortic arches. 
Car. e., External carotid. Car. i., Internal carotid. G. Gn., Ge- 
niculate ganglion. G.n. X.,Ganglion nodosum. G. pr., Gan- 
glion petrosum. ot., Otocyst. p. A., pulmonary artery. Th., 
Thyroid. v. P. 1, 2, 3, 4, First, second, third, and fourth visceral 
pouches. 
V, VU, VII, IX, X, XII, Cranial nerves and ganglia. 
The fourth visceral pouch connects with the ectoderm at its 
dorsal end, about the 35s stage, but no cleft develops. Its pos- 
terior wall develops an evagination (postbranchial body) which 
by some is considered to be a rudimentary fifth pouch, and 
which contributes to the formation of the thymus. (See 
Chap. X.) 
