OPHIURA BREVISPINA 417 



This condition of the nuclei and cell contents of the differenti- 

 ating mesenchyme cells is not that which they would assume 

 were they the initiators and self sufficient performers of their 

 change in position but is that which might be expected if the 

 organism as a whole is the dominating agent in the process. 



It is not contended that these mesenchyme cells are entirely 

 passive agents in the processes which result in their transfer 

 from the wall of the blastula to the segmentation cavity, they 

 being parts of the organism, but that the part they may take 

 in bringing about their transfer is so much less than that played 

 by the organism as a whole that it is entirely masked in this 

 case. There is no indication whatever that these mesenchyme 

 cells are wandering or migrating autonomously into the segmen- 

 tation cavity. 



In eggs containing a comparatively small quantity of yolk 

 material, the mechanical processes required to bring about 

 the transfer of mesenchyme cells take place under much less 

 complicated conditions than in an egg, such as that of Ophiura, 

 in which the quantity of inert yolk material is relatively so 

 large in comparison to the quantity of active living substance 

 associated with it. In the latter case the transfer of the mesen- 

 chyme cells can not take place until a relatively large amount 

 of inter-cellular adhesion and friction has been overcome, hence 

 processes not evident in eggs of smaller siize and different com- 

 position here become magnified and apparent. 



Working mainly with blastulae developing from eggs small 

 in size and poor in yolk content, the numerous investigators 

 of the mechanics of mesenchyme formation and invaginate 

 gastrulation have discovered within the differentiating cells, 

 or properly assumed to be present, several local factors, some 

 opei ating in one case, another series in other cases, among which 

 may be enumerated; differential growth and cell division, lique- 

 factions, absorption of liquids, intracellular migrations, changes 

 in permeability, surface tension, etc., and no one has been able 

 to find conclusive evidence of the participation of any other 

 more general differentiating activity. The demonstrated exist- 



