THE EMBRYO OF A GIBBON IN THE THIRD STAGE. 



127 



The Embryo of a Gibbon in the Third Stage. 



The embryo to be described was obtained from a female Hylobates concolor 

 in Borneo by Selenka. It still had traces of the primitive streak, the anterior 

 end of which was an open neurenteric canal. The medullary plate was partially 

 differentiated from the embryonic shield. It was thoroughly studied by Selenka, 

 and is the best known very early stage of any primate. It is more advanced than 

 the human embryo described by Peters (page 123). The entire ovum is repre- 

 sented in figure 60. The figure was reconstructed from a study of the sections. 

 It shows the chorionic membrane studded with villi. The diameter of the cho- 

 rion was about 8.5 mm. The number of villi was about one hundred, of which 

 some seventy are clustered about the region where the embryo was found. The 



Emb. Am. 



Am. EC. 



Ent. 



Yk. Ve. All. 



FIG. 61. EMBRYO OF A GIBBON, SIDE VIEW OF 



THE EMBRYO OF FIG. 60. 

 Emb, Embryo. Am, Amnion. neu, Neurenteric 



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



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



Selenka. ) 



FIG. 62. 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.) 



others are scattered over the surface of the membrane. They are considerably 

 branched. Each one is covered by ectoderm which consists of two layers, an 

 inner distinctly cellular, and an outer one in which the cell-boundaries are indis- 

 tinct and which is known, therefore, as a syncytium and represents the remains of 

 the original trophoblast. Each villus contains a core of mesodermic tissue. 

 The chorionic membrane is represented as open in order to show the size and 

 position of the yolk-sac, Yk, and of the amnion, Am, which encloses the embryo 

 as it rests upon the yolk-sac. The embryo itself is not shown in the illustration. 

 Both the yolk-sac and the amnion are, of course, covered by a layer of mesoderm. 

 The entire space between these two inner structures and the chorion corresponds 

 to the extra-embryonic coelom, the very precocious and enormous development 



