no 



EARLY EMBRYOLOGY OF THE CHICK 



the caudal portion of the embryo is not yet completely turned 

 on its side. In four-day chicks the entire body has been 

 turned through 90 degrees and the embryo lies with its left side 

 on the yolk (Fig. 40). 



myelencephalon 

 ganglion IX 

 visceral cleft II 



aortic arch IV 



ant. cardinal v. 



hyoid arch 



hyomand ibular cleft 

 mandibular arch 

 ganglion V 



metencephalon 



appendage bud t ' 



V 



vitelline artery 



posterior 

 appendage bud 



i'*iG. 39. — Dextro-dorsal view ( X 14) of entire chick embryo of 36 somites 

 (about three days incubation). 



Flexion. — The cranial and cervical flexures which appeared 

 in embryos during the second day have increased so that in 

 three-day and four-day chicks the long axis of the embryo shows 

 nearly right-angled bends in the mid-brain and in the neck 

 region. The mid-body region of thi'ee-day chicks is sUghtly 

 concaved dorsally. This is due to the fact that the embryo 

 is still broadly attached to the yolk in that region. By the 

 end of the fourth day the body folds have undercut the embryo 

 so it remains attached to the yolk only by a slender stalk. 

 The yolk-stalk soon becomes elongated allowing the embryo to 

 become first straight in the mid-dorsal region, and then convex 



