TUBULATION OF ORGAN-FORMING AREAS 



475 



grows forward as the head outgrowth (figs. 220, 223, 225). A cylindrical, 

 epidermal covering for the entire head, in this manner, is produced as the 

 cranial or brain portion of the neural plate folds inward (invaginates). A 

 similar outgrowth in the tail area proceeds posteriorly, although here the 

 neural tube grows caudally by proliferative activity within the epidermal tube 

 instead of folding into the epidermal tube as it does in the cephalic outgrowth 

 (figs. 223, 225). Coincident with these two outgrowths, the trunk area, with 

 its ventral, yolk-filled, entodermal cells, elongates antero-posteriorly as the 

 neural plate folds inward. It also grows larger in harmony with the head and 

 tail outgrowths. As these activities continue, yolk substance is used up, and 



NEURAL FOLDS 



Fig. 229. Early stages of tubulation of neural and epidermal organ-forming areas 

 with resultant body-form development in the shark, Squalus acanthias (drawn from pre- 

 pared slides). Neural area shown in black; epidermal area is stippled white; neural folds 

 are outlined in white around edges of black area. (Consult also fig. 230.) (A) Embryonic 

 area is raised upward; neural plate is flattened; bilateral tail outgrowths are indicated. 

 (B) Embryo is considerably elevated from extra-embryonic blastoderm; brain area is 

 much expanded; trunk region of neural groove is pronounced. (C) Neuralization is 

 considerably advanced; tail rudiments are converging. (D) Neural and epidermal areas 

 are well tubulated; tail rudiments are fusing. (E) Young Squalus embryo, lying on left 

 side; tail rudiments are fused into single caudal outgrowth. The body now consists of 

 a flexed cephalic outgrowth, trunk region, and tail outgrowth. (F) Squalus embryo of 

 about 10 mm. in length. 



