128 



THE HUMAN EMBRYO. 



nch. 



of which is a special characteristic of primates, including man, and is not at 

 present known to be paralleled by the conditions in the early stages in any other 

 mammals. 



A side view of the embryo on a larger scale is represented in figure 61. The 

 embryo is connected with the chorion by a well-marked body-stalk, b. s, is cov- 

 ered by the arching amnion, Am, and rests upon the yolk-sac, which in compari- 

 son to the chorionic sac seems very small. The yolk-sac, Yk, already has de- 

 veloped from it a network of blood-vessels, Ve, which contain blood-corpuscles, 

 but have not yet developed into .am embryo itself. The disposition of these ves- 

 sels is best illustrated by the section (Fig. 62). The yolk-sac is, of course, lined 



in its interior by entoderm. It has formed already 

 a prolongation, All, into the body-stalk. This 

 prolongation is the anlage of the future allantois. 

 Figure 63 represents a surface view of the same 

 embryo, or perhaps one should say, rather, of 

 the embryonic shield. At the posterior end there 

 is the short primitive streak, the anterior limit of 

 which is marked by the opening of the neurenteric 

 canal, neu, which passes obliquely downward and 

 forward, as shown also in figure 61. From the 

 end of the neurenteric canal there extends forward 

 a slight thickening of the entoderm which can be 

 recognized as the anlage of the notochord, nek. 

 Figure 62 represents a transverse section through 

 the region of the notochord. It shows the amnion , 

 Am, arching over the embryo, the thickened ecto- 

 derm of the embryonic shield, and the anlage of 

 the notochord, Ch. The mesoderm, mes, of the 

 embryo no longer extends across the median 

 line, and is without any ccelom. At the edge 



of the embryo the mesoderm splits and one layer passes over on to the 

 amnion, the other on to the yolk-sac. In the wall of the yolk-sac, D, one 

 can easily distinguish a layer of the entoderm, Ent, and also in the mesodermic 

 portion the young blood-vessels, Ve. Comparison with a section of a some- 

 what older embryo of another gibbon, Hylobates Rafflesi, also described by 

 Selenka, will be found instructive. The relations are here similar to those 

 shown in the section just described, although the stage is somewhat more 

 advanced, for we see that the amniotic cavity is larger, that the formation of the 

 medullary groove has begun, that the ccelom is beginning to appear in the em- 



pr.s 



FIG. 63. SURFACE VIEW OF THE 

 EMBRYONIC AREA OF THE OVUM 

 SHOWN IN FIG. 61. 



pr.a, Primitive axis, neu, Neurenteric 

 canal, nch, Notochord. pr.s, Primi- 

 tive streak, b.s, Body-stalk. 



