No. 3-] AMPHIOXUS AND THE MOSAIC THEORY. 6ll 



short time a half -development, although no other half exists; 

 or to that of the frog and ctenophore, in which the half-develop- 

 ment continues up to a very late period ? Roux endeavors 

 to explain these facts by assuming that the separation of the 

 blastomeres acts as a stimulus to the inactive accessory idio- 

 plasm; that the latent period following the stimulus varies; 

 and that upon the duration of the latent period depends the 

 time at which regeneration takes place. Roux is logically 

 compelled to assume the existence of accessory idioplasm by 

 committing himself in advance to the theory of qualitative 

 nuclear division. If, however, we discard this theory, the 

 assumption of an accessory idioplasm is as unnecessary in the 

 case of Echinus as in that of Aniphioxiis . It is only a state- 

 ment of fact to say that in Echinus the blastomeres of the 

 2-celled stage have become in some way more highly modified 

 than in Amphioxus, — this is proved by their behavior when 

 isolated, — and this initial modification influences the subse- 

 quent divisions for some time. If we conceive this modification 

 to affect the idioplasm, we may say that the stimulus of the 

 operation is not in this case sufficient to cause an immediate 

 return of the idioplasm to its original state, and its normal 

 equilibrium is only gradually restored through subsequent 

 cellular interaction. This is admittedly not an explanation 

 but only a re-statement of the fact. But it is at present 

 inexplicable under any theory why the reaction of the blasto- 

 meres of the 2-celled stage should differ in the two cases; 

 just as it is inexplicable why the two animals should differ 

 in any other respect. We can only say that the difference 

 depends on a difference of organization which, in turn, ulti- 

 mately depends on the nature of the original germ plasm. 

 The character of any reaction is determined no less by the 

 nature of the body stimulated than by the nature of the 

 stimulus.^ 



1 The fact (p. 593) that displaced % blastomeres of Amphioxus sometimes show 

 in their mode of cleavage reminiscences of the normal (entire) development may- 

 be taken to indicate that the extent of modification in the 2-celled stage varies 

 in different individuals. Whether the idioplasm shall at once completely rejert 

 to the original form or not may depend on the extent of this change, and this in 

 turn on the time that has elapsed between the formatio7t of the blastomeres and their 



