406 OUTLINES OF CHORDATE DEVELOPMENT 



TV. 



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 



of about flexure ) appears just back 

 of the gill cleft region, 



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

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

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

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

 and fourth visceral arches. 



as 



FIG. 166. Human embryo 

 twenty-three days (4.0 mm.). From Minot 

 (Laboratory Text-book of Embryology), 



marked curvature ap- 

 pears, so that head and 

 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. 



