446 TREADWELL. [Vol. XVII. 



and to a considerable portion of the ectoderm of the head. 

 Essentially the same fate has been described for it in Nereis 

 and Arenicola. In Podarke the cell 2d divides at first very- 

 much as in the other forms, but from the first, as was to be 

 expected from its small size, a much smaller amount of the 

 trochophore arises from it than from the corresponding cells in 

 the other genera. In the later stages, when the trochophore 

 is completed and begins to elongate, only a narrow band of 

 ectoderm near the posterior end has arisen from 2d, all of the 

 dorsal ectoderm in front of this having been derived from cells 

 which migrated through the dorsal break in the prototroch 

 (PI. XL, Fig. 59). Not only in amount, but in rate of develop- 

 ment, is 2d in Podarke less than in the other forms. Making 

 all allowances for the slow development due to unfavorable 

 conditions, there is, I think, no doubt that Podarke develops 

 the products of these cells, viz., dorsal ectoderm of the larva 

 and trunk ectoderm, more slowly than does Amphitrite. If 

 study of the pelagic larvae should subsequently prove that this 

 slow development is really due to the unfavorable conditions 

 surrounding the embryos in confinement, the other part of the 

 argument, that only a very small portion of the body is com- 

 posed of these cells, still retains its force ; but the rapid devel- 

 opment of the entoderm cells, when compared with Amphitrite, 

 would indicate, I think, that the enfeebling force of environ- 

 ment is really not responsible for the slow development of 

 ectodermal tissue. 



Again, 4d is large in unequal, small in equal cleavage. This 

 fact I believe to be correlated with the small amount and slow 

 development of mesoderm derived from 4d in Podarke when 

 compared with other forms. As already described (see p. 431), 

 the germ bands derived from 4d arise very late in the develop- 

 ment, and their cells divide slowly, so that at a time when these 

 bands in Amphitrite contain a great many cells there are in 

 Podarke only two on a side. This slow development of meso- 

 derm is also characteristic of Lepidonotus (No. 22). Further, 

 in Podarke the greater part, if not all, of the mesoderm of the 

 trochophore is composed of the larval mesoderm arising from 

 the third quartette of ectomeres ; hence the definitive mesoderm 



