No. 3] POD ARK E OBSCURA VERRILL. 427 



any trace of this cell lying in the segmentation cavity, and all 

 the larval mesoblast of the anterior end of the body, as is 

 unmistakably shown by the specimens, comes from the cell 

 3a2.2.2. I have been unable to trace the cell 3b2.2.2 with any 

 accuracy after the stage shown in PI. XL, Fig. 51, but from its 

 position I suppose that it aids in the formation of the stomo- 

 daeum. The entoderm cells have divided repeatedly, and at 

 this time are no larger than the cells at the edge of the blasto- 

 pore, so that it is difficult to distinguish between the two sets. 

 The further history of the other descendants of 3a and 3b, so 

 far as I have followed it, is given in PI. XXXIX, Figs. 46 and 

 47, and in the table, p. 438. Detailed description would be 

 profitless. They form a part of the stomodaeum and the gen- 

 eral ectoderm of the subumbrella. 



Larval Mesoblast. 



This arises in Podarke, as we have seen, from descendants of 

 3d, 3c, and 3a. The two former are symmetrically arranged 

 from the beginning, while the latter only become so placed 

 after invagination and division. It is important to note that 

 this division, which occurs in 3a2.2.2, after its invagination, cor- 

 responds to the division which in the other cells gave rise to 

 the small cell ventrally, and which takes place at the surface. 

 In both cases both products of the division become mesoblast, 

 so that all the cells of the larval mesoblast belong to the same 

 generation. I have indicated these cells as right, l.m.r., left, 

 l.m.l., and median, l.m.m. After the next division, immediately 

 to be described, the smaller product of the posterior larval mes- 

 oblast, when seen from the side, lies over the large 4d2 cells 

 (PI. XL, Fig. 56), while the others have spread farther apart 

 in the cleavage cavity. 



The larger products of the posterior larval mesoblast divide 

 next, and there is formed a band of three cells on either side 

 of the median line (PI. XL, Figs. 52 and 54, and Text-Fig. 7). 

 Since the posterior end of each band lies very close to the defin- 

 itive mesoblast (colored a deeper pink in the figures), the effect 

 is that of two well-developed mesoblast bands, lying in the 



