134 



THE DEVELOPMENT OF THE CHICK 



H.EAm, 







S.16. 



Pr.atr: 



Fig. 71. — Entire embryo of 16 s, drawn 

 from above as a transparent object. Note 

 the cranial flexure; the rotation of the 

 head on its left side is beginning, 

 au. P., Auditory pit. F. B., Fore-brain. 

 H. B. 1, First division of the hind brain. 

 H. F. Am., Head-fold of the amnion. Hm. F., 

 Hyomandibular furrow. Pr'am., Proam- 

 nion. M. B., Mid-brain, op. Yes., Optic 

 vesicle, pr. str., Primitive streak, s 2, s 4, 

 s 16, Second, fourth, and sixteenth somites. 

 V. o. m., omphalo-mesenteric vein. ^TI-V^I, 

 The acustico-facialis primordium. IX-X, 

 Primordium of the glossopharyngeus and 

 vagus. 



The entire trunk tends also to bend ventrally, i.e., to develop 

 a dorsal convexity, and this approximates its posterior end to the 

 tip of the head. These flexures are characteristic of amniote 



