EMBRYOLOGY 



Yolk-sac 



Amnion 



Allantois in body-stalk 



Notochord 



Amnion 



Neurenteric canal 



Primitive streak 



FIG. 14. Surface view of embryo of Hylobates concolor. (After Selenka.) The amnion has been opened to expose 



the embryonic disk. 





IV 



FIG. 15. Series of transverse sections through the embryonic disk of Tarsius. (After Hubrecht ) Section / passes 

 through the disk, in front of Hensen's knot and shows only the ectoderm and entoderm. Sections II, III, and IV pass 

 through Hensen's knot, which is seen in V tapering away into the primitive streak. In III, IV, and V the mesoderm 

 is seen springing from the keel-like thickening of the ectoderm, which in III and IV is observed to be continuous into 

 the entoderm. 



