REARRANGEMENT OF PRESUMPTIVE STRUCTURES DURING GASTRULATION 63 



Fig. 27. A photograph of the living gastrula of 

 a salamander. The dark, curved line is the blastopore, 

 and the region above the line constitutes the dorsal 

 lip of the blastopore. The light region below the 

 blastopore is presumptive endoderm, whereas the 

 region above the blastopore is chiefly notochord. The 

 cells at the sides of the blastopore will form meso- 

 dermal structures. 



is a long, narrow, hollow tube. It is evident that an extensive migration and 

 rearrangement of cells of the gastrula must take place in order for them to 

 assume their final positions in the embryo. 



Rearrangement of the presumptive 

 structures during gastrulation 



During the process of gastrulation some of these areas must invaginate, 

 or fold inward, to form the internal structures. We can best illustrate this 

 process by showing the position of structures before and after gastrulation. 

 At this time we shall not attempt a complete description of gastrulation 

 but shall limit ourselves to the few essentials necessary for an analysis of the 

 significance of gastrulation. A diagram of the early gastrula (Fig. 28) shows 

 a large cavity or blastocoele and the beginning of invagination at the dorsal 

 lip of the blastopore. If we take the dorsal lip of the blastopore as a marker 

 and number the regions above the dorsal lip from 1 through 5, using letters 

 A through F for the region below the lip of the blastopore, we can identify 

 these same regions at the end of gastrulation when the archenteron is 

 formed and the blastocoele has almost disappeared. It is found that region 



