250 Edmund B. Wilson. 



nificant fact, proved by these experiments in connection with ob- 

 servations by many observers of normal cell-lineage, is that the 

 germ regions prelocalized in the unsegmented egg are, at least 

 in the case of certain cells, accurately marked off by the subsequent 

 lines of cleavage. This is shown with great clearness by the his- 

 tory of the lower polar area in DentaUum (or the analogous 

 lower green area in Myzostoma or the lower polar ring in Rhyn- 

 chelmis), which, although it lies primarily at the center of the 

 lower hemisphere, is not bisected by the first or the second vertical 

 cleavage (despite the fact that both the cleavage furrows first lie 

 exactly in the egg-axis), but is moved to one side so as to pass 

 bodily into one of the cells at each division. Here is an adjust- 

 ment, of admirable accuracy, by which a specific prelocalized area 

 is handed on from cell to cell, to be finally assigned to its proper 

 positon in the cell-mosaic; and if such be the case with one such 

 specific germ area, we have strong ground to infer that it is also 

 so with others. In such cases as these it is evident that the fac- 

 tors of cleavage run so accurately parallel to those of differentia- 

 tion that they must be referred to a common determining cause, 

 and may be treated as practically identical. 



But even in cases where the adjustment is less evident, or less 

 precise (as appears to be the case, for example, in the third cleav- 

 age of the echinoderm egg, considered beyond) we shall not, I 

 believe, escape the conclusion that cleavage involves a definite 

 distribution of specific morphogenic factors among the cleavage 

 cells. The facts, proved by my experiments, that these factors 

 may be completely separated and isolated by cell-division, and 

 may retain their specific character after isolation of the cells, are 

 only intelligible under the assumption that they are somehow in- 

 volved in specific materials or stuffs which differ in a definite way 

 and have a specific topographical grouping in the undivided egg. 

 This conclusion is not to be avoided by assuming that the visible 

 cytoplasmic differences are only an accompaniment or consequence 

 of an invisible ulterior structure or organization. Admitting this, 

 and even admitting, for the sake of argument, that the localized 

 cytoplasmic factors are not definitely characterized chemical ma- 

 terials, but only local physical or structural conditions, established 



