10 



Growth in length of the Vertebrate Embryo 



but in which gastrulation can be said to be produced by invagina- 

 tion in a very small degree only. 



Gastrulation is produced by a splitting among the segments, 

 caused by a differentiation of an upper layer of small segments 

 above a layer of larger segments. We have seen that in the 

 advanced stage of segmentation the blastula of Rana has a many- 

 layered wall of which the upper part consists of smaller cells and 



Fig. 6. Frog's egg. Segmentation. 



A, 8 cell stage; B, 16 cell stage; C, 32 cell stage; D, stage with segmentation 

 cavity (sg) fully developed. 



the lower of larger cells, Fig. 6 D, with a zone of more or less 

 abrupt passage between the two ; this will be seen in sections to 

 vary slightly according to the radius along which the sections are 

 cut. 



At this stage, on the surface, one sees that the upper pole is 

 formed by small black cells and the lower pole of whiter and larger 

 cells. There is no sharp line between the two, and yet there is 

 sufficient difference to enable one to speak of an advance of the 



