A PRELIMINARY EXPERIMENTAL ANALYSIS 



41 



and can be altered by experimental means (see fig. 149 and 

 P- 320). 



In Amphibia, it has been found that the act of gastrulation can be 

 analysed into a number of component processes. First, there is the 

 tendency on the part of the cells of the animal hemisphere to ex- 

 pand and cover a larger surface. Next, the cells which constitute 

 the marginal zone between the animal and vegetative hemispheres 

 tend to stretch downwards towards the vegetative pole. This is 

 accomplished by rearrangement of the cells, with the result that the 

 ring-shaped band, increasing in depth, attempts to decrease in 



a b 



Fig. 13 

 The expanding growth-tendency of the presumptive epidermis of the Urodele 

 embryo, a. Two ventral gastrula-halves grafted together, the epidermis of each 

 of which is thrown into ridges and folds in vainly trying to overgrow the other. 

 b. The same, 16 hours later, showing intensification of ridges and folds. (From 

 Spemann, Arch. Entzvmech. cxxiii, 193 1.) 



diameter. Thirdly, the cells just beneath the marginal zone in the 

 dorsal meridian have the tendency to invaginate and form a pit-like 

 depression. Normally, of course, all these processes take place 

 together, with the result that the excess of material obtained by the 

 stretching of the marginal zone becomes tucked into the invagina- 

 tion round the rim of what may now be called the blastopore. New 

 material, as it arrives at the rim, becomes tucked in, and this 

 tendency to roll or tuck in is also an independent process. Mean- 

 while, the space vacated on the surface by the material which has 

 thus been invaginated, is occupied by the shifting and expanding 

 regions of the animal hemisphere. 



