210 



EMBRYOLOGY 



" f gyt-gi ANTERIOR NEUROPORE 



NEURAL FOLD "i;lj> 



INTESTINAL PORTAL 



SOMITE ,-' ., ; ' Hffi" 



AREA PELLUCIDA 





PRIMITIVE STREAK *rrt 



NOTOCHORD 



'» ^vNFT area va sculosa 



Fig. 124. An embryo at about 25 hours of incubation with about 7 pairs of 

 somites. Beginning with the anterior neuropore, note the enlarged forebrain 

 with suggestions of the optic vesicles; the intestinal portal, which opens into 

 the foregut; and the open neural tube with its neural folds and the paired meso- 

 dermal somites. Between the neural folds the deeper lying notochord is visible. 

 Posterior the primitive streak is seen. The embryo is surrounded by an area 

 pellucida and farther out by the area vasculosa. In it the dark spots are blood 

 islands in the process of conversion into blood vessels. These vessels will con- 

 nect with the veins and arteries which are forming within the embryo. (Courtesy 

 Mr. Lester A. Barter, Sloan-Kettering Institute.) 



section through the brain and foregut. Seven pairs of somites have formed 

 in the mesoderm, and the notochord forms a rod running from the head 

 region to the primitive streak. The streak still retains the general structure of 

 the 16-hour primitive streak previously described. 



The area pellucida has elongated considerably, and many blood islands 

 are visible in the area opaca, forming a network which may be termed the 



