THE SPIRAL TYPE OF CLEAVAGE. 



257 



meres. The early division of these cells in Arenicola, where 

 the cells do not become ciliated for a long time, must, I think, 

 be considered as without significance for the prototroch itself, 

 but as directly connected with the overgrowth of the entomeres 

 by the ectomeres. In Stcrnaspis the divisions of the cells 

 which correspond to the trochoblasts occur at almost exactly 

 the same time relatively to 

 the other cells, but here there 

 is no prototroch, and these 

 cells apparently form merely 

 a part of the ectoderm. 



The mesoblast in Ai'enicola, 

 though larger than most other 

 cells in the ^%g, divides very 

 slowly. This is not wholly 

 because of the fact that it 

 becomes differentiated only 

 at a late stage, for the ante- 

 rior ends of the mesoblast 

 bands form muscles in the 

 trochophore. But there is no room for the products of 

 its division in the blastocoele in early stages (Fig. 24). If it 

 divided rapidly, invagination of the entomeres would be impos- 

 sible, unless the ectoderm were stretched to a considerable 

 degree. In cases where the mesoblast is smaller, the rapidity 

 of division may be more closely connected with the time at 

 which it becomes functional as mesoblast. 



The size of the various cells is another factor. It is appar- 

 ently correlated with several features of the process of morpho- 

 genesis, viz., the final fate of the cells in question, the stage at 

 which the material is used, and the relation to other cells in 

 the complex. Lillie and Conklin have called attention to the 

 first two of these factors, but the third seems fully as important 

 as either of the other two. The size of the various cells is 

 an essential factor in the accomplishment of the processes of 

 growth. Examination of the growth of the somatic plate gives 

 the impression that not only is the direction of division 

 adapted to the form of growth, but the size of each cell also. 



Fig. 24. — Arenkola. Mesoblasts ^O/ nearly fill- 

 ing the blastocoele. Optical section. 



