372 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



influence exerted upon the diverted piece in Stenostoma. 

 In both cases there is a remarkable display of the inherent 

 formative power of the organism, but with Stenostoma this 

 comes more prominently to view in that what is accom- 

 plished here is done by this power acting more independ- 

 ently, with less external aid than it has in the embryos. 

 There is no reason assignable to external influences why 

 the normal fission should not continue after the operation and 

 separate off wholly the piece a.f.p. In fact, this gener- 

 ally does happen, and when it does the derived worm (P. T.) 

 is as complete in every respect as when it retains a. f. p., 

 and is never of reduced size as appears to be always the 

 case when a larva is produced from an embryo from which 

 one or more of the blastomeres have been removed. 



(b) As to the Causes of the Migration of the Gangli- 

 onic Cell-Mass. — If the facts here presented are questioned 

 relative to the nature of the formative forces of organisms, 

 which we want so much to get hold of, the reply that comes, 

 while of course in no sense final, does, it seems to us, 

 strongly confirm the conception that, whatever their ulti- 

 mate nature, they must be regarded in their relation to 

 the organism as a -whole. 



Sachs' conception of " specific formative substance" and 

 his generalization that all differences in the form of organs 

 are correlated with differences in their chemical constitu- 

 tion, and that the principles of science require us to suppose 

 the form-differences to be derived from the chemical differ- 

 ences,^ probably represents the best that biological science 

 can do at present in this direction toward the solution of the 

 problem of development. But this conception is wholly 

 incapable of yielding much light on the problem in detail, 

 unless the reciprocal action of form and form-changes upon 

 chemical constitution receives adequate recognition. 



For example, we may appeal to some tropism, probably 

 cytotropism, with much confidence, in view of recent studies 

 on this subject, particularly by Herbst (1894) and Roux (1894) 



1 Quoted by Loeb, 1893, p. 55. 



