CLEAVAGE IN ANNELIDS. 



IOI 



the corresponding quartette, while in Podarke these cells are 

 no larger than corresponding cells in the other quadrants. 



Fig. 7, of Amphitrite, and Fig. 8, of Podarke, sho\v the upper 

 poles of embryos in practically the same stages as Figs. 5 and 6. 

 Note here the comparatively large size of the cross-cells in 

 Podarke, composing a larger portion of the umbrella than do 

 the corresponding cells in Amphitrite (cf. Figs. 7 and 8). 



Leaving the question of strictly cell development, let us pass 

 to the later embryonic history and note how the trochophore 

 arises from the embryo of sixty-four cells. The differentiation 

 of the prototroch cells enables us to distinguish an upper 



FIG. 7. Amphitrite, about sixty-four cells 

 from above ; ap-dp, primary trochoblasts. 



FIG. 8. Podarke, forty cells from above ; 

 ap'-dp ', trochoblasts before division . 



umbrella from a lower subumbrella region. Later, the proto- 

 troch ring is partially completed in Amphitrite by the addition 

 of three cells in each of three quadrants from the lower hemi- 

 sphere, the fourth, that of D, remaining open for some time. 

 Through this opening a number of cells from the upper hemi- 

 sphere invaginate and form a portion of the subumbrella ecto- 

 derm. The descendants of 2d, or X, have meanwhile multiplied 

 rapidly, and compose the greater part of the trunk ectoderm. 

 As I said before, Mead believes that all of the trunk ectoderm 

 behind the first segment is composed of the descendants of 

 these cells. Meanwhile certain of them become ciliated and 

 form the paratroch, a band of cilia incompletely surrounding 

 the body just in front of the anus. At this time, when the 



