No. 3.] PODARKE OBSCURA VERRILL. 447 



appears late in the development, and in accordance with 

 our theory its Anlage in 4d is small when formed. So far, 

 then, our principle holds good for 4d. 



We have already seen that the larval mesoderm of Podarke 

 arises asymmetrically, and that 3b does not contribute to its 

 formation. Objection might be made to the above theory on 

 the ground that, if it were true, since 3b does not contain 

 larval mesoblast, it should be smaller than 3a, which does. I 

 believe, however, that 3b contributes a larger amount of mate- 

 rial to the stomodaeum than does 3a. As described on p. 426, a 

 cell very similar in size to the mesoblast arising from 3a is 

 formed at about the same time as the latter in 3b, and I at 

 first supposed that it contributed to the mesoblast formation. 

 Careful examination, however, failed to show any trace of it in 

 the segmentation cavity, and I believe that its fate is as above 

 described ; so that a larger part of the stomodaeum arises from 

 3b than from 3a. 



We have now accounted for the small size of 2d and 4d, and 

 this explanation carries with it the explanation for the equality 

 of D with the other cells in the 4-cell stage. There remains 

 to explain the reason for the equality between micromeres and 

 macromeres in the 8-cell stage. This may be due in some meas- 

 sure to the small amount of yolk, but the comparatively large size 

 of the micromeres is, I believe, correlated with the unusually 

 large development of the umbrella as compared with the sub- 

 umbrella of the trochophore, and with the fact that a large portion 

 of subumbrellar ectoderm really comes from the first quartette 

 of micromeres. (See p. 413.) In discussing this point in a pre- 

 vious paper (No. 29, b) I overlooked the fact that the proto- 

 troch cells of Amphitrite are relatively much larger than those 

 of Podarke, and I argued that the relatively larger cross cells 

 in the latter would account for the relatively larger size of the 

 first group of micromeres. The larger size of the prototroch 

 cells in Amphitrite would probably compensate for the small 

 cross, and so the argument loses its force ; but the further 

 consideration, that a very large part of the subumbrellar ecto- 

 derm in Podarke arises from these cells, is in itself a sufficient 

 explanation for their size. We have already seen that the 



