HUMAN EMBRYO IN THE FIFTH STAGE. 



131 



ectoderm and entoderm. Although the extra-embryonic coelom is fully devel- 

 oped, that of the embryo is present as a small fissure, />, only. Figure 66 is 

 a section passing through the neurenteric canal, and shows, therefore, the 

 amnion, am, the thickened medullary plate, e, of the embryo, and the large 

 yolk-sac, d. The yolk-sac is formed, of course, of splanchnopleure. The thick- 

 ening of the mesodermic layer in the lower part of the yolk-sac in order to 

 allow space for the developing blood-vessels, 6, 6, 6, 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 neuren- 

 teric canal, come nearer together in the 

 region 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 re- 

 ceive a venous branch from the caudal 

 side of the yolk-sac. 



Fig. 67. — Human Embryo with Open Medullary 



Groove. 

 Am, Amnion, b.s. Body-stalk. Cho, Chorion. Md, 

 Medullary folds. Yk, Yolk-sac— (^//^r W. His.) 



Human Embryo in the Fifth Stage with Open Medullary Groove. 



Although several embryos in this stage have been studied, none of them has 

 furnished very thorough information. The two best studied were recorded by 

 His; one he designates as " E" and the other as " SR " (Fig. 67). The chorionic 

 vesicle of "E" measured 8.5 X 5.5 mm.; of "SR," 9X8 mm. The embryo in 

 "E" measured (?) 2.1 mm.; in "SR," 2.2 mm. (Fig. 67). It will be noticed at 

 once that the condition is very similar to that shown in figure 65, but the embryo 

 is somewhat more advanced. The most important changes in the embryo at 

 this stage are its general growth, so that it rises above the yolk and has both 

 projecting head and projecting tail. The medullary groove is very deep and 

 extends the entire length of the embryo. Toward its caudal end it probably has 

 an open neurenteric canal. The dorsal outline of the embryo is somewhat con- 



