120 THE ORIGIN OF THE NERVOUS SYSTEM 



The apparent independence and capacity for "self- 

 differentiation" of different levels or regions of the 

 body in certain animals, e.g., annelids and mollusks, 

 may seem to be completely irreconcilable with the 

 conclusion advanced above, but what the data on "self- 

 differentiation " and "mosaic development" actually 

 demonstrate is not the absence of relation between 

 parts but merely the fact that protoplasmic specializa- 

 tion has advanced so far that the regulatory reactions 

 to isolation of parts are limited, and the behavior of the 

 part after isolation is therefore much the same as before. 

 The evidence indicates that conditions of this sort are 

 due simply to early cytoplasmic specialization rather 

 than to the fundamentally "mosaic" character of 

 ontogeny. 



THE PHYSIOLOGICAL INDEPENDENCE OF APICAL REGION 

 AND HEAD IN EXPERIMENTAL REPRODUCTION 



In normal ontogeny the whole body is potentially 

 present, and objection to the conclusions stated above 

 may be raised on the ground that, even where differen- 

 tiation is not perceptible, it may still be sufficient at 

 lower levels of the axis to influence the development of 

 higher levels. But further evidence in support of these 

 conclusions concerning independence and dependence, 

 some of it demonstrative, is obtained from the regula- 

 tory development of isolated pieces from many of the 

 lower animals. In the first place it has long been known 

 that in the development of pieces isolated from the body 

 of various species of hydroids, flatworms, and annelids, 

 the apical or anterior region of the pieces gives rise 

 directly to an apical structure or head, irrespective of 



