TUBULATION OF ORGAN-FORMING AREAS 



489 



BULBUS COnDtS_ 



MESENCHYMe OF HEAD' 



Fig. 239. Sections through chick embryo of age indicated in fig. 238. Level of sections 



is shown on diagram. 



(See Chap. 22.) In the elasmobranch fishes, this retraction of tissues 

 contributes Uttle to the formation of the wall of the enteron or to that 

 of the body. However, in teleosts such contribution is considerable. 



At this point reference should be made to figures 238C on the chick, 242C 

 and G on the pig, and 245B on the early human embryo to gain a visual image 

 of the developing foregut, midgut, and hindgut areas of the primitive meten- 

 teron. Compare with the frog (fig. 225C). 



