FROM TWELVE TO THIRTY-SIX SOMITES 177 



small round dorsal, and long fissure-like ventral cleft at about 

 the 40 s 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. 



Fig. 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- 

 gHon petrosum. ot., Otocyst. p. A., pulmonary artery. Ih. 

 Thyroid, v. P. 1, 2, 3, 4, First, second, third, and fourth visceral 



pouches. _ . , , 1- 



V, VH, VIII, IX, X, XII, Cranial nerves and ganglia. 



The fourth visceral pouch connects with the ectoderm at its 

 dorsal end, about the 35 s 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 

 w^hich contributes to the formation of the thymus. (See 

 Chap. X.) 



