No. 4.] CELL LINEAGE OF PODARKE OBSCURA. 203 



of Lepidonotus. Amphitrite, on the other hand, rapidly 

 develops strong parapodia with numerous setae, and by sixty 

 hours has four trunk segments. Compare also the four-day 

 trochophore of Eupomatus 1 (Hatschek's Fig. 50, which strongly 

 resembles the Podarke trochophore) with the sixty-hour trocho- 

 phore of Nereis (Wilson's Fig. 91). The former is thin-walled 

 with a large cavity and no trace of metameric segments or of 

 parapodia. The latter has three pairs of segments, with large 

 parapodia, setae, and cirri. If the law that the size of a cell 

 bears some definite relation to the size and time of appearance 

 of the organ to which it gives rise 2 applies in other cases, may 

 it not apply here as well, and may we not suppose the extra 

 amount of material stored in cell D of Nereis, Amphitrite, etc., 

 is in some way related to the need for an extra amount of 

 somatic and mesoblastic material in the young larva ? 



I think that the supposition is a reasonable one, and would 

 state my conclusions thus. The large size of the posterior 

 macromere D in forms with unequal cleavage is due to the fact 

 that the young larvae of these forms require for their develop- 

 ment an excessive amount of the characteristic products of this 

 cell, - -mesoblastic and somatic tissue, and hence arises an 

 accumulation of this material in this particular cell. This 

 material differs, not in quality, but in quantity from that in the 

 corresponding cell in equal cleavage (hence there is as truly a 

 "precocious segregation" in the one case as in the other), and 

 the two cells are to be regarded as absolutely homologous. 



MARINE BIOLOGICAL LABORATORY, 



WOODS HOLL, MASS., 



Aug. 23, 1897. 



1 Hatschek, " Entwick. d. Trochophore von Eupomatus." Wien, 1885. 



2 Cf. Lillie, /. c., and Conklin, /. c. 



