10 WILSON. 



certain, first whether the pigment-cells are in part identical with 

 or descended from the small cells budded forth from the 

 "primary mesoblasts," and second, whether, if this be the fact, 

 the cells of such origin also wander in to form a part of the 

 entoblast, A careful study of the successive stages in surface 

 views, optical sections, and actual serial, sections hardly leaves 

 room for doubt in regard to either point. In the first place, 

 pigment is developed in the small cells that abut directly 

 against the primary mesoblasts (Fig. 3, /^), and the products 

 of the latter form so considerable a group that it would hardly 

 be possible to overlook their displacement or wandering away 

 did such a process occur before the appearance of the pigment. 

 I can find no evidence of such displacement and hence cannot 

 escape the conclusion that the pigment-cells lying just anterior 

 to the primary mesoblasts have been derived from them. The 

 evidence on the second point, while perhaps not demonstrative, 

 is hardly less convincing. The pigment-cells disappear from 

 the surface pari passu with the growth of the archenteron ; and 

 w^hen the latter is fully formed (in embryos of five days and 

 upwards) not a trace of pigment can be found at the surface or 

 in any of the cells of the posterior region save those of the 

 archenteron. That the superficial pigment-cells actually pass 

 inwards is proved by the fact that from its first appearance the 

 pigment is densest in two (sometimes three) symmetrical areas 

 which are first seen at the surface and may then be traced pro- 

 gressively inwards in the archenteric wall.^ 



Taken together, these facts leave no doubt, in m}' opinion^ 

 that the pigment-cells are derived in part from the primary meso- 

 blasts, in part from the entomeres, and that the cells from both 

 sources give rise to a portion of the archenteric wall and to no 

 other structure. If this conclusion be correct, it follows that 

 the " primary mesoblasts" are not properly so-called, but are 

 mesentoblasts, precisely as Conklin has described in Crcpidida. 

 Now, there can be no doubt that the single pair of minute cells 

 in Aricia and Spio represent the group of cells of like origin in 



1 Cf. 1892, Figs. 79-91, which show this fact, through not as clearly as it appears 

 in my more recent preparations. 



