. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 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
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. 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
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