ZOOL.— Vol. II.] RITTER-CONGDON—STENOSTOMA. 373 



to explain the migration of the ganglionic cell-masses to 

 their new position in the worms made up of P. T. -f- a.f. -p. 

 This tropism would have to be supposed to rest ultimately 

 on some physico-chemical peculiarity of the cell-masses. 

 What is this peculiarity and how does it come about? It 

 may be conjectured that the cut, not very far in front of the 

 masses, in some way, possibly by putting the tissues of the 

 cut surface into more intimate contact with the water and 

 its contained oxygen, may have produced changes of this 

 nature requisite to bring about the migration; but this ex- 

 planation would appear to be negatived by the fact that it is 

 usually not until after about twenty-four hours, i. e., after 

 the cut has had time to heal up and so exclude the water 

 and give the tissues more nearly their original character, 

 that the migration begins. Were this explanation the right 

 one it would seem that the migration should take place soon 

 after the wound is made, while the conditions supposed to 

 cause it would be most fully present. Nor can we appeal 

 to a specific physico-chemical change set up through 

 hereditary causes ; for never before in all the phylogenetic 

 history of Stenostoma has its braiii migrated. The journey, 

 and it will be noted that it may be a rather long one, the 

 length depending on how far forward a. c. is placed, is 

 made now for the first time. It looks as though XhQprinmm 

 movens in the operation, even though we assume some 

 tropism to be the immediate cause, is the specific form of 

 the animal. 



We may conceive all the tissues of the individual animal 

 to be in a state, not of e^?^/-librium but of Stenostoma-X-CQ- 

 rium ; and that when this is disturbed in any way the whole 

 system together tends to re-establish it; and this may be 

 done i^/zr(9?^^/2 physico-chemical means. Loeb's conclusion 

 (1899) that the number of embryos produced from a single 

 ovum is dependent upon the geometrical form of the ^gg 

 substance, would seem to be well taken, not only for the 

 particular phenomena to which it relates, but so far as con- 

 cerns the potency of form, as touching a general principle 

 that is very wide reaching. 



