FROM EARLY NEURULA TO TAIL-BUD EMBRYO 125 



move toward the midline, as indicated by the arrows in Figure 66. The 

 boundary of the sector nearest the dorsal lip moves toward the blastopore 

 during gastrulation. As a result of the combined movements of cells the 

 presumptive neural plate becomes rearranged into an elongated structure 

 somewhat wider at one end. The wider portion of the neural plate becomes 

 brain, while the long, narrow region forms the spinal cord. During this re- 

 arrangement the area of the neural plate increases considerably at the expense 

 of a decrease in its thickness. 



The various rearrangements of groups of cells during gastrulation have 

 sometimes been described by a general term, invagination. Strictly speaking, 

 however, invagination is a folding process and gastrulation in the amphibian 

 egg involves more than folding. The formation of the floor of the archenteron 

 by an inward folding of the yolk cells (Fig. 64) is a process of invagination. 

 The roof of the archenteron forms by a combination of two processes. First, 

 there is a migration of cells to the lip of the blastopore, where they turn 

 under the lip to form the roof (regions 1 through 4 in Figs. 62 through 65). 

 The turning-under process is called involution. Second, the blastoporal lips 

 migrate over the surface of the vegetal hemisphere, bringing B closer and 

 closer to C. This migration over a surface is termed epiboly. Gastrulation, 

 therefore, is a process in which some invagination, some involution, and some 

 epiboly are involved. No one term adequately describes the entire process. 



From early neurula to 

 tail-bud embryo 



The series from 36 hours to 58 hours (Fig. 67) is characterized by the 

 appearance of the neural plate and its transformation into a neural tube and 

 primitive brain. Stage 13 shows the neural plate faintly outlined as a thicken- 

 ing of the ectoderm. By stage 14 the edges of the plate have become elevated 

 into neural ridges, while the middle of the plate is relatively depressed. The 

 folding process continues, and in stage 15 the neural folds have contacted 

 each other and begin to fuse, forming a tube. In stage 16 the folding is 

 complete and a long, narrow neural tube widening anteriorly into the brain 

 has developed. By stage 17 the line of fusion of the neural ridges has almost 

 disappeared. The formation of the neural tube from the neural plate in the 

 salamander is illustrated in Figure 68. 



