136 



THE HUMAN EMBRYO. 





pleure. The thickening of the mesodermic layer in the lower part of the yolk-sac 

 in order to allow space for the developing blood-vessels, b, b, b, is well shown in the 

 figure. 



Eternod has studied an embryo in this stage. He finds that the heart is 

 already present underneath the slightly projecting head. From its anterior end 

 it sends out two aortic branches which run on either side near the notochord, pass 

 in a gentle curve around the neurenteric canal, come nearer together in the region 



.<' 



Ent 



All. 



Yk. 



b.-- 



FIG. 80. HUMAN EMBRYO or 1.54 MM. MEDIAN SEC- 

 TION FROM A WAX MODEL RECONSTRUCTED FROM 

 SECTIONS. 



All, Allantois. Am, Amnion. b.s, Body-stalk. Cho, 

 Chorion. EC, Ectoderm. Ent, Entoderm. mes, 

 Mesoderm. Vi, Chorionic villus. Yk, Cavity of 

 yolk-sac. (After Count Spec.) 



FIG. 81. HUMAN EMBRYO OF i .54 MM. 

 Transverse section passing through the neu- 

 renteric canal and yolk-sac, am, Amnion. 

 ek, Ectoderm. ct, Amniotic mesoderm. 

 g, Meeting-point of somatopleure and 

 splanchnopleure. df, Mesoderm of yolk- 

 sac, b, b, b, Blood-vessels, en, Entoderm. 

 n, Neurenteric canal, d, Cavity of yolk- 

 sac, e, Medullary plate. (After Count 

 Spec.) 



of the primitive groove, and enter the body-stalk, through which they run parallel 

 to the allantois and form ramifications in the chorion. He finds also two veins in 

 the body-stalk which, when they reach the embryo, unite to a single median trunk, 

 which quickly divides into two vessels which run in the mesoderm of the yolk-sac 

 near the embryo proper until they reach the venous end of the heart, into which they 

 open. They each receive a venous branch from the caudal side of the yolk-sac. 



Human Embryo in the Fifth Stage with Open Medullary Groove. 



Several embryos in this stage have been studied. Two have been studied by 

 W. His; one he designates as "E" and the other as "SR" (Fig. 82). The 

 chorionic vesicle of "E" measured 8.5 X 5.5 mm.; of "SR, " 9X8 mm. More 



