STRUCTURE OF FOUR-DAY CHICKS 



III 



dorsally. At the same time the caudal flexure is becoming more 

 pronounced. The progressive increase in the cranial, cervical, 

 dorsal, and caudal flexures results in the bending of the embryo 

 on itself so that its originally straight long-axis becomes 

 C-shaped and its head and tail lie close together (Fig. 40). 



myelencephalon 

 visceral arch III 

 bulbo-conus-arteriosus 



auditory vesicle 



endolymphatic duct 

 ganglion IX/ / ganglion VII-VIII 



hyomandibular cleft 

 mandibular arch 



ganglion V 



cephalon 



anterior 

 appendage bud 



omphalo- 



mesenteric 



vein 



border of 

 mesonephros 



posterior appendage bud 



FIG. 40. Dextral view of entire chick embryo of 41 somites (about four days 



incubation) . 



The Visceral Arches and Clefts. A fourth visceral cleft has 

 appeared caudal to the three that were already formed in 55- 

 hour chicks. The visceral arches are thicker and more conspicu- 

 ous than in earlier embryos. In lightly stained whole-mounts 

 of a three-day chick it is still possible to make out the aortic 

 arches running through the visceral arches. In a chick of four 

 days the visceral arches have become so much thickened that it 

 is very difficult to see the vessels traversing them. 



The Oral Region. The cervical flexure presses the pharyn- 

 geal region and the ventral surface of the head so closely to- 

 gether that it is difficult to make out the topography of the oral 



