444 HOLMES. [Vol. XVI. 



the hypothetical shifting process of Driesch. The blastomeres 

 are not shifted about promiscuously, but cells occupying simi- 

 lar positions to those in the unreversed eggs contain cytoplasm 

 derived approximately from the same portion of the ovum ; 

 and it may be for this reason, and not on account of the mere 

 fact of position, that they come to have the same destiny. Cor- 

 responding portions of the egg cytoplasm develop along similar 

 lines, whose direction appears to be independent of the precise 

 form of cleavage, even when the cleavage is of a highly deter- 

 minate type. The direction of every cell division may be 

 reversed up to a late period of cleavage without altering the 

 fate of the cells having the same position in the q^^. 



Reversal of cleavage, like the pressure experiments of Driesch 

 on echinoderm eggs, and of Wilson on the eggs of Nereis, 

 shows that the immediate causes of the differentiation of cells 

 lie, not in the nucleus, but in the cytoplasm. While certain 

 cells have the same position and fate in reversed and unreversed 

 eggs, the nuclei of these cells have entirely different lines of 

 descent ; and, conversely, nuclei having the same origin come 

 to lie in entirely different portions of the embryo. Thus, in 

 perfectly normal development, the fate of cells appears to be 

 entirely independent of the origin of their nuclei. How this 

 fact can be reconciled with the view that the differentiation of 

 blastomeres is mainly the result of qualitative nuclear divisions, 

 I cannot understand, unless we suppose that there is some com- 

 plex mechanism for the proper sorting of nuclear material to 

 provide for the contingency of reversed cleavage. 



