REARRANGEMENT OF CELLS DURING GASTRULATION 



123 



PRESUMPTIVE 

 EPIDERMI 



PRESUMPTIVE 

 EURAL PLATE 



PRESUMPTIVE 

 NOTOCHORD 



Fig. 65. The end of gastrulation. The archenteron is large and the blastocoel 

 very small. The yolk cells are covered by the overgrowth of the ectoderm, except 

 for a small region, the yolk plug. 



The rearrangements of all the cell areas during gastrulation are somewhat 

 difficult to follow and therefore in Figure 66 a detail of the form changes 

 in the presumptive notochord is shown. On the surface of the early gastrula 

 the presumptive notochord spreads out in the shape of wings with its narrow- 

 est border near the blastoporal lip. Arrows serve to indicate the direction of 

 movement. As gastrulation proceeds, part of the presumptive notochord 

 moves over the lip of the blastopore. In so doing it becomes compressed into 

 a long, narrow strip of tissue. Later this strip separates from adjacent tissues 

 and becomes a long, narrow rod of cells between the neural tube and the 

 primitive digestive system. As the notochord migrates over the dorsal lip it 

 comes in contact with the presumptive neural plate and induces the latter to 

 differentiate into the neural plate. 



Gastrulation also modifies the shape of the presumptive neural plate. It 

 begins as a sector of a hollow sphere. The cells in the angles of this sector 



