134 THE DEVELOPMENT OF THE CHICK 
Pr. str-——_ ——— 
Fig. 71.— Entire embryo of 16s, drawn 
from above as a transparent object. Note 
the cranial flexure, also the rotation of the 
head on its left side. 
au. P., Auditory pit. F. B., Fore-brain. 
H. B. 1, First division of the hind brain. 
H.F. Am., Head-fold of theamnion. Hm.F., 
Hyomandibular furrow.  Pr’am., Proam- 
nion. M. B., Mid-brain. op. Ves., Optic 
vesicle. pr. str., Primitive streak. s 2, s 4, 
s 16, Second, fourth, and sixteenth somites. 
V.o.m., omphalo-mesenteric vein. VII-VIII, 
The acustico-facialis primordium. IX—X, 
Primordium of the glossopharyngeus and 
vagus. 
The entire trunk tends also to bend ventrally, 7.e., to develop 
a dorsal convexity, and this approximates its posterior end to the 
tip of the head. These flexures are characteristic of amniote 
