ADAPTATION IN CLEAVAGE. 



55 



FIGS. 6-1 1. Cleavage of a number of annelids, showing progressive increase in the size of the 

 trunk-forming cells, viz., the first and second somatoblasts, d 2 = A' and d* = M. The first 

 somatoblast, which forms the ectoderm of the trunk, is indicated throughout by horizontal 

 lines, and the second somatoblast, which forms the mesoderrn of the trunk, by vertical 

 lines. The endoderm-cells are ruled with lines crossing at right angles. The stippled 

 cells are the primary trochoblasts, destined to form the prototroch. i. First generation 

 of ectomeres ; 2. Second generation of ectomeres; 3. Third generation of ectomeres. 



FIG. 6. Thirtv-two-celled stage of Podarke from the left side. Taken, with the kind permis- 

 sion of Mr. Treadwell, from one of his unpublished drawings. 



FIG. 7. A iiifihitrite, after Mead. From the left side and below. 



FIG. 8. Nereis limbata, after E. B. Wilson, from the left side. 



FIG. 9. Clymenella torquata. after Mead, from the left side. 



FIG. 10. Aricia, optical longitudinal section, after E. B. Wilson. 



FIG. ii. Clepsine, after C. O. Whitman. From the animal pole. The great size of the eudo- 

 derm-cells is due to yolk. The somatoblasts far exceed in bulk all the other cells. 



