406 OUTLINES OF CHORDATE DEVELOPMENT 



where they reunite into the dorsal aorta. A small anterior 

 carotid artery appears a little later. From the dorsal aorta is 



given off a pair of small 

 vitelline arteries supply- 

 ing the yolk-sac, while 

 posteriorly the dorsal 

 aorta divides into a pair 

 of umbilical or allantoic 

 arteries, passing through 

 the body stalk and sup- 

 plying the allantois. 



As the head now en- 

 larges rapidly a well- 

 marked flexure (cervical 

 flexure) appears just back 



FIG. -166. Human embryo of about 



twenty-three days (4.0 mm.). From Minot of the gill cleft region, 



(Laboratory Text-book of Embryology), -, -i ,-> i .1 , 



after His (Embryo a). al, Fore-limb bud; while through the entire 



BS, body stalk; Op, optic vesicle; pi, hind- body and tail a strongly 

 limb bud; IV, fourth ventricle of brain; 1, 



mandibular process; 2, hyoid arch ; 3, 4, third marked CUTVatlire 



and fourth visceral arches. . hea( j 



Op 



pi 



peaj . g> gQ 



tail are almost brought in contact, and the entire embryo is 



almost circular in general 



outline (Fig. 166). The fore- 



and hind-limb buds have ap- 



peared as low, extended ele- 



vations, and the abdominal 



region is becoming promi- 



nent on account of the 



growth of the liver and 



other viscera. The ventral 



body wall now becomes more 



completely closed together, 



and the tissues (somatopleu- 



ral) at the base of the body 



stalk grow forward enclosing 



the root of the yolk Stalk for FIG. 167. Human embryo of about 9.3 

 ,. . mm. After Hochstetter, X 6 2/3. 



a Short distance, SO that a For description see text. 



