228 Edmund B. Wilson. 



corresponding group in a whole embryo (Cf. Fig. 5), though 

 these divisions are often (as in all the foregoing cases) less un- 

 equal than in a whole embryo, and their arrangement is fre- 

 quently modified by shifting of the cells. At the end of 24 hours 

 these groups produce closed ovoidal or irregular ectoblastic vesi- 

 cles that swim rather slowly by means of a tuft of cilia at one 

 end. In some cases these cilia seem to be borne by a single cell; 

 in others I am sure there are two of these cells (Fig. 82). These 

 cells are evidently secondary trochoblasts; and their presence is 

 entirely in agreement with the facts observed in the J^-macro- 

 mere-larva described under ( i ) , and with the fact that the nor- 

 mal larva clearly shows the derivation of at least one, and prob- 

 ably two, secondary trochoblasts from the second quartet. 



There are two additional noteworthy points in these larvse. 

 One is the fact that, in addition to the one or two secondary 

 trochoblasts, some of them, at least, show one or two other small 

 patches of short and feeble cilia like those seen in the }i or tV 

 macromere larva. If my interpretation of these cells is correct, 

 this may be taken as evidence that the ventral ciliated tract arises 

 from derivatives of both the second and third quartets. 



A noteworthy point in these embryos is the presence, in some 

 of them, of loose groups of rounded cells lying within the cavity 

 (Fig. 82). These cells, considerably smaller than the entoblast- 

 cells, are not improbably mesenchyme cells of the "larval mesen- 

 chyme" (paedomesoblast or ectomesobast) ; and it may here be 

 recalled that in Crepidiila, according to Conklin ('97), the ec- 

 tomesoblast is derived from the second quartet. Soon after the 

 stage described the embryos died and disintegrated without fur- 

 ther noticeable change, 



4. Observations on isolated cells obtained from larvae that 

 have developed' continuously in calcium-free water. 



Beyond the facts recorded above, I have not traced the devel- 

 opment of isolated blastomeres from the lower hemisphere. Sev- 

 eral times I succeeded by successive isolation in separating single 

 cells of the third and fourth quartets, and of the corresponding 



