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[VOL. I. 



this division is dexiotropic, the form which it ought to assume 

 under the law of alternating cleavages. A distinction between 

 the anterior and posterior arms can be recognized during several 

 divisions of the cross cells and until meridional divisions of its 

 cells destroy the outline of the cross. 



At the vegetative pole the next division is a bilateral one, of 

 one of the fourth group of micromeres, accompanied by a 

 bilateral division of the lower members of the third group 

 which lie on either side of it. (See Fig. 5.) Here c" 2 and d 3 ' 2 

 have divided, while d* is still in process of karyokinesis. Just 



ffr 



FIG. 3. Optical section of embryo at the first division after the 64-cell stage. 

 Ar., apical rosette. En., entoderm. 



above this cell is the small cell .r 1 ' 2 , and a plane passing through 

 these cells coincides with the plane of bilateral symmetry of 

 the cross. 



This cell of the fourth quartette is d 4 = M\ and, while the 

 other three members of the fourth quartette migrate bodily 

 into the segmentation cavity, its products remain at the sur- 

 face until the blastopore has nearly closed. They then divide, 

 each sending a cell into the segmentation cavity, and form the 

 mesoblast bands. 



The inner ends of the entoderm cells A 4 , B 4 , C 4 , D 4 elongate 

 very considerably, and the nuclei migrate to a considerable dis- 

 tance toward their inner end. (See Fig. 3, which shows the 



