482 DEVELOPMENT OF PRIMITIVE BODY FORM 



growth and extension of the posterior or tail region of the primitive archen- 

 teron of the late gastrula. These changes result in an extension of the 

 archenteron in the direction of the developing tail and the area ventral to the 

 tail (compare fig. 220B-D with figs. 223B, C; 225B, C). 

 Three general areas of the primitive gut are thus established: 



(a) a tubular enlargement and outgrowth into the developing head, the 

 primitive foregut, 



(b) a tubular extension and growth in the caudal region toward the tail, 

 the primitive hindgut, and 



(c) a midgut area whose ventral wall is filled with yolk substance, while 

 its roof or dorsal wall assumes a trough-like form extending below 

 the notochord (figs. 223, 224, 225). 



The foregut and hindgut areas at this time present the following special 

 features: 



( 1 ) Two terminal diverticula or evaginations evolve at the extreme anterior 

 portion of the foregut; and 



(2) at the extreme caudal end of the hindgut, similar evaginations occur. 



In the foregut region, one of these evaginations projects toward the brain 

 and anterior end of the notochord, while the second diverticulum, more pro- 

 nounced than the dorsal evagination, moves ventrad toward the epidermis 

 underlying the developing brain. The dorsal evagination represents the pre- 

 oral or head gut. In the frog it is much abbreviated (figs. 220B, C; 225B, C). 

 On the other hand, the antero-ventrally directed, oral, or pharyngeal, evagi- 

 nation is relatively large and projects toward the ectoderm underlying the 

 brain where it forms the future pharyngeal area of the foregut (figs. 220; 223; 

 225B, C). Ultimately an invagination from the epidermis, the stomodaeum, 

 becomes intimately associated with the anterior end of the pharyngeal evagi- 

 nation (see Chap. 13). In the hindgut region, the diverticulum which projects 

 dorsally into the tail is the tail gut, whereas the ventral evagination toward 

 the epidermis below the tail represents the future rectal and cloacal areas of 

 the hindgut (figs. 220; 223; 225B, C). It shortly becomes associated with 

 an invagination of the epidermis, the proctodaeum (fig. 223B, C). As previ- 

 ously mentioned, the tail gut may be well developed, as in the European frog, 

 Bombinator (fig. 228), or quite reduced, as in the frog, Rana (fig. 225). 



c. Formation of the Tubular Metenteron in Flat Blastoderms 



The development of the cylindrical gut tube in those vertebrate embryos 

 which possess flattened gastrulae is an involved, complicated affair. The de- 

 velopmental mechanics are not clearly understood. For example, it is not 

 clear whether the embryonic layers, lying in front of the head fold in figure 

 232G and H, are folded slightly backward in figures 232K and L and still farther 



