EARLY DEVELOPMENT IN THE FROG. 351 



such, e.g., as the flatworms, it appears that as the body increases 

 in length the posterior region comes to lie beyond the range of 

 dominance (control) of the apical end. As a consequence, the 

 posterior end undergoes much the same changes that it would 

 if physically isolated. It begins to dedifferentiate and its suscep- 

 tibility increases, i.e., it comes to resemble physiologically 

 younger tissue. If these processes go far enough the posterior 

 region gives rise to a new zooid which in time may become a new 

 individual. 1 



In the annelids such a region arises early in development but 

 instead of producing zooids which develop into independent indi- 

 viduals it proceeds to give rise to segments that are reintegrated 

 and brought under control, probably through the specialization 

 of the nervous system. From this point of view the development 

 of the annelid trunk appears to be a succession of incomplete 

 reproduction processes, but which undergo reintegration into a 

 whole consisting of segments. 



In the light of all the facts it is difficult to avoid the conclusion 

 that the origin of the dorsal lip region is fundamentally similar 

 to these processes of physiological isolation, since the dorsal 

 lip region arises secondarily at the posterior end of the embryonic 

 area as a region of high susceptibility, increased activity, and 

 rapid growth. 



The secondary imaginations afford an interesting bit of evi- 

 dence in that, with certain degrees of inhibition of the apical 

 region, the range of dominance is decreased and after a first iso- 

 lation has occurred a second one may arise nearer the apical end. 



3. The Question of "Specificity" in Teratogeny. The word 

 "specificity" as used in the literature on vertebrate teratogeny 

 is difficult of exact definition. In general it has been used to 

 designate some unique reaction of an organism or part, to a 

 particular experimental factor, especially a particular chemical 

 substance, such as to produce an anomaly characteristic of 

 that individual external factor. 



Various attempts have been made to identify experimentally 

 produced abnormalities with either a specific chemical effect or 

 with a more general physical effect of some one of numerous 



1 Child, 1911, Jour. Exp. Zoo!., Vol. n. 



