THE EMBRYO OF A GIBBON IN THE THIRD STAGE. 



133 



primates, including man, and is not at present known to be paralleled by the con- 

 ditions in the early stages in any other mammals. 



A side view of the embryo on a larger scale is represented in figure 76. The 

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

 by the arching amnion, Am, and rests upon the yolk-sac, which in comparison to 

 the chorionic sac seems very small. The yolk-sac, Yk, already has developed from 

 it a network of blood-vessels, Ve, which contain blood-corpuscles but have not yet 

 developed into the embryo itself. The disposition of the vessels is best illustrated by 

 the section (Fig. 77). 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 78 represents a surface view of the 



Emb. Ant. 



b. s. 



Am. EC. F. 



Ent: 



Yk. 



All. 



FIG. 76. EMBRYO OF A GIBBON, SIDE VIEW OF 

 THE EMBRYO OF FIGURE 75. 



Emb, Embryo. Am, Amnion. neu, Neurenteric 

 canal, b.s, Body-stalk. Yk, Yolk-sac. Ve, 

 Blood-vessels. All, Allantois. (After E. 

 Selenka.) 



FIG. 77. TRANSVERSE SECTION OF THE EMBRYO 

 OF THE PRECEDING FIGURE. 



Am, Amnion. EC, Ectoderm. F, Dorsal furrow. 

 mes, Mesoderm. Ent, Entoderm. Ve, Blood- 

 vessel. (After E. Selenka.) 



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 down- 

 ward and forward, as shown also in figure 76. 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, nch. Figure 77 represents a transverse 

 section through the region of the notochord. It shows the amnion, Am, arching 

 over the embryo, the thickened ectoderm of the embryonic shield, and the anlage 

 of the notochord. 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 meso- 

 derm 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 ento- 

 derm, Ent, and also in the mesodermic portion the young blood-vessels, Ve. Com- 

 parison with a section of a somewhat older embryo of another gibbon, Hylobates 



