476 DEVELOPMENT OF PRIMITIVE BODY FORM 



the ventro-lateral region of the trunk is retracted. A cyHndrical shape of the 

 trunk region thus is established, bringing the trunk area into harmony with 

 the head and tail outgrowths. (Study particularly fig. 227.) The epidermal 

 area of the late gastrula thus becomes converted into an elongated, epidermal 

 tube which forms the external covering or primitive skin (see Chap. 12) jor 

 the developing body. In Amphibia, this primitive epidermal tube is two layered, 

 consisting of an outer epidermal ectoderm and an inner neural ectoderm (figs. 

 221, 224). (See Chap. 12.) In the newly hatched larva, the epidermis is 

 extensively ciliated in all anuran and urodele Amphibia. 



b. Tubulation of the Epidermal Area in Flat Blastoderms 



In the flat blastoderms of the elasmobranch fish, chick, reptile, and mammal, 

 the formation of the external body tube involves processes more complicated 

 than that of the frog type. The following steps are involved: 



( 1 ) A head fold produces a cephalic epidermal extension above the gen- 

 eral tissues of the blastoderm. This rudimentary fold of the epidermis 

 contains within it a similar fold of the entodermal layer, together with 

 the invaginating, neural plate material. The notochordal rod lies be- 

 tween the forming entodermal fold and developing neural tube (figs. 

 213F; 230A; 232I-L; 242B, C). Shortly, the primitive head fold 

 becomes converted into a cylindrical head outgrowth of the epidermal 

 and entodermal layers, associated with the forming neural tube and 

 notochord (figs. 229C, D; 230C; 233). The general process is similar 

 to that in the frog, but it is more complicated in that the head rudiment 

 first must fold or project itself up above the extra-embryonic areas, 

 before initiating the outgrowth process. 



(2) A second procedure involved in epidermal tubulation in flattened 

 blastoderms is the dorsal upgrowth movement of epidermal, meso- 

 dermal, and entodermal tissues. This activity hfts the trunk region of 

 the embryo up above the general blastodermic tissues (figs. 213H-J; 

 234B; 241). In some forms, such as the chick, the dorsal upgrowth 

 movement is more pronounced in the anterior trunk area at first, 

 gradually extending caudad to the trunk region later (figs. 233, 235). 

 However, in the pig, human, and shark embryos, the dorsal elevation 

 extends along the entire trunk area, coincident with the head out- 

 growth, and thus quickly lifts the embryonic body as a whole up 

 above the extra-embryonic tissues (figs. 229, 230, 242, 245). 



(3) The tail outgrowth, in reptiles, birds, and mammals, begins in a manner 

 similar to that of the head region, and a tail fold first is developed 

 which later becomes a cylindrical projection, bounded externally with 

 epidermal cells, within which are found the notochord, tail mesoderm, 

 and tail portions of neural and gut tubes (figs. 238C; 239K, L; 245B). 



