PHYSIOLOGICAL BASIS OF PATTERN 35 



are more susceptible superficially than in 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 cer- 

 tain concentrations Mr. McArthur has found that 

 susceptibility of the median ventral region is distinctly 

 greater than that of dorsal and lateral regions. The 

 early embryonic stages of Planaria have not been 

 available for work on susceptibility, but the data on 

 other flatworms and various other bilateral inverte- 

 brates 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, 1913^) it was noted that the relations 

 as regards median, lateral, and dorsal were not entirely 

 clear. As regards the alimentary tract, various facts 

 indicate that in the adult the region of greatest activity 

 and highest susceptibility is in the middle of the body 

 about the base of the pharynx and that a gradient of 

 decreasing activity extends in both directions from 

 this region, but since the alimentary tract is an internal 

 organ and is not readily separable from other parts it is 

 difficult to obtain conclusive evidence on this point. 

 The anteroposterior gradient of ectoderm and body 

 wall persists throughout life, but with the appearance of 

 new zooids at the posterior end new gradients arise in 

 that region, or, more strictly speaking, the original gradi- 

 ent undergoes modification. Apparently the primary 



