298 



THE DEVELOPMENT OF THE CHICK 



which the two tubo-tympanic cavities open. The median cham- 

 ber communicates by a longitudinal slit (tubal fissure) in the 



roof of the pharynx with the 

 oral cavity (Figs. 168 and 

 175). 



The frontal partition in 

 question is a posterior pro- 

 longation of the palatine 

 processes of the maxillary 

 arch, and forms as follows: 

 If the head of a four-day 

 chick be halved by a sagit- 

 tal plane, and the interior 

 of the pharynx and mouth 

 cavity be then viewed by 

 reflected light, an elongated 

 lobe will be seen on the me- 

 dian surface of the mandib- 

 ular arch and maxillary 

 process (Fig. 174 A). This 

 lobe begins far forward on 

 the median surface of the 

 maxillary process and may 

 be followed posteriorly over 

 the median surface of the 

 mandibular arch to the first 

 visceral pouch, where it 

 ends with a free rounded 

 extremity. The lobe itself 

 is called by Moldenhauer 

 the colliculus palato-phar- 



O.PhT 



GoDjJ.p. 



Fig. 174. — A. Head of a chick embryo of 

 4 days, halved by median section and 

 viewed from the cut surface. (After 

 Moldenhauer.) 



B. Internal view of the pharynx of a 

 pigeon embryo, corresponding in develop- 

 ment to a chick of 10 days. (After Mol- 

 denhauer.) 



Col. 1., Colliculus lingualis. Col. p. p., 

 Colliculus palato-pharyngeus. Cr. i., Crus 

 inferior. Cr. s., Crus superius. Hyp., 

 Hypophysis. Mx., Maxilla. N'ch., No- 

 tochord. O. Ph. T., Ostium tubse phar- 

 yngae. S. P., Seessell's pocket. 2, 3, 4, 

 Second, third, and fourth visceral arches. 



yngeus; it is bounded above 

 and below by depressions, 

 viz., the sulcus tubo-tym- 

 panicus dorsally and the 

 sulcus lingualis ventrally, 

 both of which end behind 



in the first visceral pouch; 

 anteriorly the ventral furrow disappears at the margin of the 

 mouth, and the dorsal furrow near SeessePs pocket. The maxil- 



