6o8 WILSON. [Vol. VIII. 



immediately changes if the blastomere be isolated. Thus at 

 the third normal cleavage of Amphioxus (first unequal division) 

 each blastomere divides unequally because it is one of four that 

 have a certai7t relation to 07ie another ; for if one of them be 

 separated from its fellows of the 4-celled stage it invariably 

 divides equally and not unequally.^ In this case, therefore, it 

 must be the interaction of the cells that determines their 

 specific form of development, whether the nature of the inter- 

 action be mechanical or physiological. In terms of the idio- 

 plasm hypothesis, the idioplasm of the first two or four 

 blastomeres of Amphioxus must be fundamentally of the same 

 character, and the nature of its ensuing activity in each blasto- 

 mere depends on circumstances — that is, on the relation of 

 the cell to its fellows. This, however, is only to say that the 

 activity of the idioplasm is determined by cellular interaction, 

 as Oscar Hertwig maintains. In this case the Weismann- 

 Roux assumption of qualitative division does not harmonize 

 with the observed facts, while that of accessory idioplasm is 

 perfectly gratuitous and unnecessary. And if this is true of 

 any one stage of the ontogeny a very strong presumption is 

 created that it is true of all. 



The most significant fact in the development of isolated 

 blastomeres of AmpJiioxus is that up to the 8-celled stage 

 their power of development progressively diminishes as the 

 cleavage advances, {a) The Yi blastomere often segments like 

 a normal ovum, but not seldom shows variations some of which 

 are reminiscent of the normal cleavage-half. It gives rise to 

 a normally formed blastula and gastrula, which may develop 

 into a perfectly formed half sized dwarf larva having all of the 

 essential characteristics of the adult, {b) The % blastomere 

 may likewise segment nearly or quite like a normal &^g, but 

 in general varies more from the normal type than the ^ 

 blastomere. A perfect gastrula_ may be formed, but the de- 

 velopment rarely continues long beyond the closure of the 

 blastopore and then gives rise only to abnormal or defective 



1 The inequality in the normal cleavage cannot be due to an unequal distribu- 

 tion of yolk caused by gravity, for, as in Echinus, the segmentation planes show no 

 constant relation to the vertical. 



