No. 3.] PODARKE OBSCURA VERRILL. 467 



me as close as Professor Conklin would maintain, nor as close 

 as one would infer from his statement (p. 194, etc.). 



3. The X-Cells. — In the lower hemisphere the cell 2d has 

 been cited as an example of cell homology, for from it arise 

 the ectoderm of the trunk, the ventral plate, and the growing 

 point. Within the limits of the cell the most important larval 

 organ is the paratroch. The cell origin of this is known only 

 in Amphitrite and in Arenicola, but it is not the same in the 

 two cases. In Podarke, as already stated, I have been unable 

 to find any paratroch, and the comparisons which I have made 

 between the divisions of X in Podarke and those in the other 

 two annelids show absolutely no similarities. 



A further point in the development of Podarke is the rela- 

 tively small amount of dorsal ectoderm arising from 2d. As 

 shown in PL XL, Fig. 59, this forms only a comparatively 

 narrow band around the proctodaeum, all the rest of the dorsal 

 ectoderm having arisen from cells which migrated through the 

 dorsal break in the prototroch. Either, then, we must assume 

 that 2d is not completely homologous in Podarke and in 

 Amphitrite, or we must assume that the portion of the trocho- 

 phore lying just behind the prototroch is not homologous 

 in the two cases. Of the two, the former seems the more 

 reasonable. 



The small cell, X1.2, arises in the same manner and occupies 

 the same position in all the forms studied. Its fate is un- 

 known, except for the general statement that it forms a part 

 of the proctodaeal wall. The constant position of this small 

 cell must have some meaning, although I am not able at present 

 to say what that is. It may be another case of complete homol- 

 ogy, and for the sake of the argument we may assume that it 

 is, though it should be remembered this has not been proved. 



4. The Mesohlast. — There remains one more cell for which 

 a complete homology is assumed in the various forms, and that 

 is the cell 4d, which, with but one exception, contains the 

 definitive mesoblast. 



This cell arises from the posterior macromere D, and, so far 

 as its origin is concerned, is alike in all cases. That it is 

 qualitatively different is shown, I think, by the most recent 



