616 HAROLD HEATH, 



upon they pass into the interior. This is manifested by a change in 

 the leiotropic position of these cells which become strictly radial, by 

 a more compressed appearance as indicated by a polar furrow of 

 greater length, and a loss of roundness of cell contour in the exposed 

 portions , and also by the form of the cells which are clearly pear- 

 shaped with the larger end bordering the segmentation cavity. This 

 is the first step in the invagination process and may be said to commence 

 in the initial stages of the 36 cell embryo. 



The cell from' which the mesoblast arises is generally slightly 

 the largest of the macromeres. Not infrequently this is not mani- 

 fested by the external surface but in such cases it is found to have 

 migrated to some degree into the interior. After the mesoblast is 

 formed this inward movement generally becomes quite pronounced and 

 by it less and less of the external surface remains exposed. The 

 process appears as a consequence of the actual inward movement of 

 the mesoblast cell accompanied by the overgrowth of the mesoblast 

 by the posterior members of the third quartette. A comparison of 

 the figures of the early and late stages of invagination will show 

 that as the mesoblast passes in the tertiary stomatoblasts , at first 

 widely separated, gradually cover the former and in so doing ap- 

 proach each other along the antero-posterior axis. 



In the 85 cell stage the remaining members of the fourth 

 quartette form. The position of the spindles is radial and the division 

 likewise. 



Somewhat later than the preceding division spindles appear with 

 bilateral arrangement in the third quartette stomatoblasts. The 

 resulting cleavage divides the anterior cells into equal moieties, but 

 the posterior cells each bud off a small cell anteriorly that is crowded 

 in between the parent cell and the neighboring fourth quartette. 



Movements among the cells on the vegetative pole such as have 

 been mentioned are usually observed previous to the stage last 

 described, but they may be said to be premonitory since no very 

 marked invagination has occurred, the only effect being a slight 

 decrease in the external surfaces of the macromeres and the meso- 

 blast cell accompanied by a flattening of the vegetative pole. With 

 this last cleavage of the stomatoblasts invagination may be said to 

 fairly commence, but in order to understand its processes together 

 with the shifting of the blastopore it will be necessary to consider 

 in some detail the relations of the cells on the vegetative pole. 



In a stage previous to the division of the third quartette stomato- 



