418 CASWELL GRAVE 



ence of these local activities and effects has been accepted by 

 some investigators therefore as evidence that the developmental 

 processes in question are wholly autonomous and not brought 

 about or controlled by the organism as a whole 



The existence of local changes within the cells of a differentiat- 

 ing area, facilitating or contributing the movements or changes 

 in the cells, is to be expected, even though controlling or directing 

 stimuli for the migration or invagination process might be defi- 

 nitely traced to some more general source. 



Izi the mesenchyme cells of Ophiura there is also evidence 

 of the effect of some of the above mentioned local factors. A dif- 

 ferential absorption of liquid, probably from the segmentation 

 cavity, has evidently taken place provided the comparatively 

 larger size of the nuclei of these mesenchyme cells (when com- 

 pared with the nuclei of the adjacent cells of the blastula wall) 

 is accepted as proof of such absorption by the cells, as the work 

 of Glaser ('14a) indicates it may be. See figure 1. If these cells 

 absorb liquid then it is probably safe to assume that other chemi- 

 cal changes and surface effects within these differentiating cells 

 would naturally follow. 



The change which has taken place in the disposition of the 

 cytoplasmic and nuclear materials within these mesenchyme 

 cells, however, is not such as can be satisfactorily explained 

 as due to an autonomous intracellular migration. On the other 

 hand it seems to register the effect of coercive pressure. The 

 wrinkled condition of the membrane beneath the mesenchyme 

 area seems to be an especially faithful witness of such a general 

 force and it is as evident in the living as in the preserved blastula. 



Cell multiplication may m some cases play a role in supplying 

 the necessary tension for material transfers or movements 

 in development but in Ophiura, as in Sphaerechinus (Morgan 

 '95) and Amphioxus (Morgan and Hazen 1900), there is no 

 evidence although dividing cells are numerous that they are 

 so located as to be differentially efficient. 



In the section shown in figure 1, eight cells are in mitotic 

 division and in the entire blastula, from which the figured sec- 



