302 C. M. CHILD 



In the course of my experiments with very large individuals 

 during the winter of 1909 — 10, I obtained positive evidence that 

 in such animals four zooids instead of two actually exist and was 

 able in some cases to induce fission between the third and fourth 

 zooids before that between the second and third. In my earlier 

 paper (Child '10) I called attention to the probable existence of 

 four zooids in some individuals and these later experiments 

 afford further confirmatory evidence. As a matter of fact there 

 is good reason for believing that the second zooid actually con- 

 sist of two zooids in a great many cases, perhaps always, after it 

 has attained a certain size: if, for example, this second zooid is 

 cut into very small pieces we commonly find that the regional 

 distribution of the regulatory capacities resembles that in the 

 whole individual from which this zooid was taken. A decrease 

 in the power of whole formation occurs with increasing distance 

 of the level of section from the zone of fission to the posterior 

 quarter of fifth or the zooid where it increases again. In short, 

 if we cut the second zooid into safficiently small pieces it appears 

 as a miniature of the whole animal. 



The fact that the zooids in Planaria do not become visibly 

 morphologically differentiated before fission, is at least in part 

 the result of the physical consistency of the planarian tissues 

 which is such that independence of motor reaction brings about 

 separation before the degree of physiological isolation is sufficient 

 to permit the differentiation to become visible. The result of 

 inhibition of fission will be discussed at another time. 



If Driesch's view that structure is developed for function, not 

 by it (Driesch, '05, etc.), a view which he has expressed very pos- 

 itively in opposition to various observations and conclusions of 

 mine, is correct, the existence of such a region in the planarian 

 body, functioning like a second younger animal, but without any 

 of the visible morphological characteristics of such an animal 

 would seem to be impossible. Yet there can be no doubt that 

 the second zooid is there and that in certain respects and to a 

 certain degree it functions as an independent animal. I believe 

 that here as elsewhere in organisms, visible structure is merely 

 the record of past functional i. e., dynamic activity of some kind 



