Gastrulation 



11 



small black cells into the region previously occupied by the large 

 white cells. This advance is an advance of a zone of differentia- 

 tion ; it is certainly not an actual movement of black cells over 

 the white, in Rana, though it may be so in some other Amphibia. 

 Thus there is a tendency during segmentation for the black area 

 to increase at the expense of the white, for pigment is deposited 

 as the larger cells segment into smaller ones. Gastrulation is 

 first rendered evident by the appearance of a slight groove running 

 parallel to the equator of the egg but some distance below it, 

 Fig. 7. 



Fig. 7. Diagram to illustrate the external views and corresponding 

 internal changes during gastrulation in the frog. 



A, B, C, D, external views; E, F, G, H, median sections; arch archenteron; 

 bl blastopore ; dl dorsal lip of blastopore ; vl ventral lip of blastopore ; sg seg- 

 mentation cavity; in H the segmentation cavity has almost disappeared. 



We will now very briefly look at the general characters as one 

 sees them in surface view and sections, without at first considering 

 the mechanism. 



If we examine the groove in section it will be noticed that 

 there are signs of pressure, the cells are compressed and there 

 is more pigment deposited between the segments at this point 

 than elsewhere. 



If we watch the surface we find, Fig. 7 A D, that the groove 



