158 C. M. CHILD. 



ment the original gradient disappears completely, a new gradient 

 in the opposite direction arises at the opposite end, and the first 

 hydranth arises from the high end of this new gradient. In 

 other words, the phenomenon which Loeb and others have called 

 heteromorphosis is a normal feature of hydroid development. 

 Again in the polyclad turbellaria the early stages of develop- 

 ment show a very distinct antero-posterior susceptibility gra- 

 dient, the head arising from the most susceptible region, but in 

 the later stages, so far as they have been examined, a reversal 

 in the susceptibility relations occurs, at least in the superficial 

 regions, and in the adult worms the ectoderm of the head is less 

 susceptible than more posterior levels. These data on hydroids 

 and polyclads have not yet been published in full. In the an- 

 nelids a second gradient in the opposite direction from the pri- 

 mary antero-posterior gradient appears at least superficially in 

 the posterior regions as the result of the development of the 

 posterior growing region (Hyman, '16; Child, '17^). But what- 

 ever the changes, they are definite and orderly and associated 

 with the course of development in each species. 



As development and differentiation progress, the indications 

 of more or less specific relations between particular regions or 

 organs and particular agents become more frequent. In Planaria 

 dorotocephala, for example, we find that the lateral margins of 

 the body are more susceptible superficially than the median 

 regions to alkalies, while in neutral and acid agents there is little 

 difference between median and lateral or the median ventral 

 region is slightly more susceptible than dorsal and lateral. With 

 methylene blue of certain concentrations Mr. McArthur has 

 found that susceptibility of the median ventral region is distinctly 

 greater than that of dorsal and lateral regions. The early em- 

 bryonic stages of Planaria have not been available for work on 

 susceptibility, but the data on other flatworms and various other 

 bilateral invertebrates indicate that primarily the susceptibility 

 of the median ventral region is higher than that of dorsal and 

 lateral regions and the outgrowth of tissue from cut surfaces in 

 pieces of adult Planaria suggests a greater parenchymal activity 

 in the median ventral region. In the previously published data 

 on the gradients in Planaria (Child, '13^) it was noted that the 



