48 The Unity of the Organism 



actually present. 



But whatever be the cells which transform into the spin- 

 ules, all the available evidence indicates that the cytoplasm 

 is the chief source of the shield part of the spinule. The 

 possibility is not excluded that the nuclear chromatin also 

 plays some part in the development. Indeed investigations, 

 notably those by Duesberg on the ascidian egg made since 

 "cytoplasmic inclusions" have come into prominence are 

 distinctly favorable to such an hypothesis. But this is very 

 far from proving that such chromatinic influence, assuming 

 it to exist, reduces the cytoplasm to pure passivity. 



In the light of what is here set forth let us examine the 

 view expressed by Conklin and quoted above that the egg 

 cytoplasm fixes the general type of development, while the 

 nuclei of egg and sperm together "supply only the details." 

 The examination should be the more cogent from the fact 

 that Conklin's idea was based largely on his investigation 

 of ascidian eggs, some of which pertained to the very same 

 genus, Styela, as do the spinules just described. The reader 

 should not fail to notice that both Conklin and myself are 

 dealing with single cells, he having to do with egg-cells, while 

 I am concerned with spinule cells. Nor should the fact be 

 lost sight of that these two categories of cells stand at 

 about opposite extremes in the life career of an individual 

 Styela, the egg being at. the very beginning, while the spinule 

 is produced at or near the completion of adulthood. Since, 

 however, both cells possess attributes distinctive of the 

 species, there can be no more "filling in of details" at one 

 end of the series than at the other, so far as concerns the 

 differential attributes under consideration. 



Now comes the main point. The differential attributes 

 of the egg-cell and early embryo, their "polarity, symmetry, 

 and pattern," are "determined by the cytoplasm," to use 

 Conklin's own words. What is the evidence that these attri- 

 butes are thus determined? That of direct observation, 



