620 HAEOLD HEATH, 



Thus in the embryo two agencies are active in producing a 

 shifting of the blastopore; first, on the posterior side there is little 

 or no invagination after the earlier stages but a rapid increase of 

 ectodermal cells by which the area between the prototroch and blasto- 

 pore is constantly increasing ; second, on the anterior side of the egg 

 cell divisions are of less frequent occurrence, and invagination de- 

 creases the estoblastic area lying between the blastopore and the 

 prototroch thus bringing the blastopore ultimately next to the velum. 

 The final changes, after the invagination of the third quartette 

 stomatoblasts , are relatively much longer than were the movements 

 up to this period, and it appears that the first movements of in- 

 vagination up to the point where the anterior third quartette stomato- 

 blasts disappear from view are relatively so rapid because the cells 

 are crowding into the segmentation cavity. When the cells have in- 

 vaginated and the segmentation cavity is filled a more or less stable 

 equilibrium is reached. After this period for every decrease of 

 space between the blastopore and prototroch there must be a pro- 

 portionate increase in the posterior surface of the embryo. 



In order that it might not complicate the account of the in- 

 vagination to a greater degree I have left out the part the second 

 quartette plays in the process. This is the more permissible since 

 this quartette for some time does not actively participate in the 

 general movement though influenced by it in the earlier stages. For 

 example in the 113 cell stage (Fig. 28) the cells in question {2a^-'^-'^ 

 etc.) are situated without the circle of cells that will invaginate, and 

 in the stage shown in Fig. 35 they have not advanced completely 

 into that territory. 



They are now below the general level being wedged in between 

 the stomatoblasts of the third quartette. The movements of these 

 cells are essentially the same in all quadrants but differences in their 

 size cause some changes which modify the external appearance of the 

 embryo. For example the cell in the anterior quadrant is always the 

 largest, the one in the posterior normally the smallest, while the cells 

 of the right and left quadrants are intermediate in size. 



In such a stage as is represented by Fig. 50 the channel between 

 the two sets of anterior third quartette stomatoblasts is shallow 

 and never in the history of the embryo does a deep 

 groove extend from the blastopore to the prototroch. 

 In the right and left quadrants where the stomatoblasts are smaller 

 they come to lie at the bottom of comparatively deep furrows between 



