484 DEVELOPMENT OF PRIMITIVE BODY FORM 



caudad in figure 233C and D by autonomous activities within this tissue, or 

 whether the actively growing head outgrowth proceeds so rapidly that it 

 mechanically causes the area in front of the head fold to rotate backward 

 under the developing foregut and thus contribute to the foregut floor. It is 

 obvious, however, that the entodermal material, lying in front of the head 

 fold of the embryo, is folded backward, at least slightly, and thus becomes 

 a part of the floor of the foregut. The extent, however, varies considerably in 

 different species. It appears to be greater in the mammal (fig. 242C) than in 

 the chick. Another example suggesting the integration of different movements 

 of cellular layers is presented in the formation of the floor of the hindgut of 

 the developing pig embryo. In figure 242C, the rudiments of the foregut and 

 hindgut areas are established. However, in figure 242G, it is difficult to eval- 

 uate how much of the floor of the hindgut in this figure is formed by actual 

 ingrowth forward from point "a" and to what extent the floor is formed by 

 the rapid extension of tissues and backward growth of the caudal region of 

 the embryo as a whole, including the allantoic diverticulum. 



Special processes also aid the formation of foregut and hindgut in many 

 instances. For example, in the chick, the floor of the foregut is established 

 in part by a medial or inward growth and fusion of the entodermal folds along 

 the sides of the anterior intestinal portal, as indicated by the arrows in figure 

 234C. A similar ingrowth of entoderm occurs in the shark embryo (fig. 213J). 

 although here the entoderm grows in as a solid layer from either side and is 

 not present in the form of a lateral fold, as in the chick. However, it should 

 be observed that the formation of the hindgut in the shark embryo arises by 

 a most interesting and extraordinary method. In the flattened gastrulae of 

 reptiles, birds, and mammals, the hindgut is established by the formation of 

 tail folds, involving entodermal and epidermal layers. In the shark embryo, 

 on the other hand, an enteric groove with enteric folds is formed, and the 

 folds eventually move ventrad and fuse to form a hollow tube beneath the 

 notochord of the developing tail. 



Though the rudimentary foregut and hindgut areas of the metenteron arise 

 almost simultaneously in mammalian embryos, such as in the pig and human 

 embryos, in the chick a different sequence of procedure is present. In the 

 latter species the foregut begins its development immediately following gas- 

 trulation when the first pairs of somites are present (fig. 233). The hindgut, 

 on the other hand, begins its development at a considerably later period when 

 the embryo has attained many pairs of somites (fig. 238). 



Once the rudimentary, pouch-like, foregut and hindgut areas have been 

 established in embryos developing from flattened gastrulae, their further de- 

 velopment assumes morphogenetic features similar to those in the frog embryo. 

 For example, the foregut possesses an antero-dorsal prolongation toward the 

 brain, the pre-oral or head gut, while slightly posterior to the pre-oral gut, 

 the future pharyngeal area makes contact ventrally with the stomodaeai in- 



