THE EARLY DEVELOPMENT OF THE MAMMAL 405 



ot. 





Just a few words may be added concerning the internal 

 structure of this embryo (Fig. 165). The slender notochord 

 extends the entire length of the body and tail. Optic vesicles 

 are distinct and the otocysts are entirely closed off below the 

 surface. The expanded pharynx shows an anterior hypo- 

 physial outgrowth, in addition 

 to the lateral branchial pockets. 

 A small rudiment of the lung 

 is indicated, and at the pos- 

 terior end of the narrowed 

 oesophagus the liver rudiment 

 is well marked. Posterior to 

 this the gut is open into the 

 yolk-sac by way of the yolk 

 stalk, and continuing poster- 

 iorly from this is the narrow 

 intestine, which, near its ex- 

 tremity, sends a small allan- 

 toic outgrowth into the body 

 stalk. Rudiments of the 

 mesonephros (Wolffian body) 

 and its duct are slightly in- 

 dicated. 



The vascular system is very FlG - 165. Reconstruction of a hu- 



man embryo of 2.6 mm. (bee .big. 



Well developed. Opening into 164). From Minot (Laboratory Text- 

 .1 ifj.ii book of Embryology), after His. A, 



the posterior end of the heart Aortic limb of heart> Attt body stalk . 

 there are, the paired ductus A < dorsal aorta = Au < umbilical arter- 



' . ies; Car, posterior cardinal vein; Jg, an- 



Cuvien, formed by anterior terior cardinal vein (jugular vein). Om, 



vpirm omphalomesenteric vein; op, optic ves- 



IS > icle; ot, otocyst; Vh, right umbilical 



Car: 



smrl 



the paired vitelline or omphalo- vein. 

 mesenteric veins, coming from 



the yolk-sac by way of the yolk stalk, and the paired allantoic 

 or umbilical veins coming from the allantoic region by way of 

 the body stalk. Opening out of the anterior end of the heart 

 is the ventral aorta, which immediately divides into right and 

 left halves from each of which arise five aortic arches passing- 

 through the visceral arches to the dorsal side of the pharynx, 



