478 DEVELOPMENT OF PRIMITIVE BODY FORM 



In elasmobranch fishes, two flattened tail outgrowths are present at 

 first which later fuse into a single cylindrical outgrowth (cf. figs. 229; 

 230F). 

 (4) A ventral constriction of the ventro-lateral body areas, involving the 

 ingrowth of the lateral body folds, occurs in the trunk region as indi- 

 cated in figure 241. This movement aids the establishment of a cylin- 

 drical body form in the trunk region. Entodermal and mesodermal 

 body layers, as well as the epidermal layer, are concerned with the 

 ventral constrictive movement (fig. 24 IB, C). 



As a result of the above activities, an elongated, cylindrical body form is 

 effected in which the epidermal layer forms the outer covering around the 

 other body tubes. 



3. Formation of the Primitive Gut Tube (Enteric Tubulation) 



a. Regions of Primitive Gut Tube or Early Metenteron 



The details of formation of the enteric tube vary considerably in different 

 vertebrate species. However, in all, the archenteric conditions of the gastrula 

 are converted into a primitive tubular metenteron, having three main regions 

 as follows: (1) foregut, (2) midgut, and (3) hindgut. 



b. Formation of the Primitive Metenteron in the Frog 

 The formation of the foregut in the frog naturally follows as a result of 

 the anterior growth and extension of the cephalic portion of the primitive 

 archenteron present at the end of gastrulation (fig. 220B, C). This outgrowth 

 accompanies the forward growth of the neural and epidermal tubulations of 

 the developing head described above. The primitive head outgrowth thus is 

 composed of the anterior ends of the epidermal, neural, and gut tubes together 

 with the head mesoderm, all oriented around the median notochordal rod 

 (figs. 221B, C; 223B, C). 



The midgut area of the primitive metenteron forms in relation to changes 

 in the developing trunk region. At the end of gastrulation, its ventral portion 

 is filled with yolk-laden cells, while its middorsal area is occupied by the 

 median notochordal band of cells (fig. 219B, C). This middorsal area is soon 

 completed by the medial growth of the entoderm which grows inward from 

 either side below the notochord (fig. 219D). Accompanying the completion 

 of the roof portion of the midgut, the entire midgut area becomes extended 

 antero-posteriorly (figs. 220B, C; 223B, C; 225B, C). Associated with these 

 changes, the middorsal area of the midgut moves dorsad toward the notochord, 

 forming a dorsal, trough-like region of the gut (fig. 224). It is to be observed 

 in this connection that the neural tube invaginates toward the notochordal 

 rod, whereas the roof of the gut evaginates (i.e., in a sense it invaginates) 

 toward the same notochordal area. This dorsal folding of the gut tube in the 



