A 



B 



C 



D 



Fig. 172, yl-F.— Cleavage stages of Arenicola. A, two-cell stage; B, second cleavage; C, 

 eight-cell stage; D, second division of first quartet of micromeres and of the cell 2d (first somato- 

 blast) ; E, primary trochoblasts, shaded, and two-cell stage of 2d, surrounded by heavy line, 

 also the cell M {4d), the second somatoblast or mesoblast; F, later stage, showing development 

 of somatic plate, surrounded by heavy line, the two mesoblasts, outlined in broken line, al- 

 ready in blastocoel, and entoderm cells indicated by parallel broken lines (after Child, 1900). 



