112 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Differential susceptibility of planarians to many agents has been deter- 

 mined (see Appendix III, p. 734). In species without posterior zooids and 

 fission susceptibility of ectoderm and body wall, as indicated by disinte- 

 gration or other death changes, progresses from the head region posterior- 

 ly over the whole body length, except that the extreme posterior tip is 

 usually somewhat more susceptible than regions immediately anterior to 

 it, agreeing in this respect with results of dye reduction. Species develop- 

 ing posterior zooids and undergoing fission show a less simple longitudinal 

 susceptibility gradient. ^^ In these forms one or more posterior zooids are 

 more or less clearly distinguishable by their susceptibility, though not 

 morphologically. Two, three, or even more posterior zooids are often 

 present in long individuals of D. dorotocephala, and under certain condi- 

 tions fission of any one can be induced (Child, 1910a, 191 1 J). 



In gradually lethal concentrations or intensities above the limit of tol- 

 erance death of the anterior zooid progresses from the head posteriorly, 

 but lateral margins are more susceptible than the median region to alka- 

 line and irritating agents (Fig. t,^,, A-D) and less susceptible to agents 

 which kill without stimulation or irritation (Fig. ^^, E-G). The differ- 

 ences between margin and median region suggest a certain degree of spe- 

 cific susceptibility of the margins, perhaps associated with the presence 

 there of numerous gland cells; these are apparently stimulated to secre- 

 tion by alkaline and irritating agents. Dorsal and ventral epithelium and 

 pharynx also show indications of specific susceptibility. To some agents 

 the dorsal, to others the ventral, epithelium is more susceptible, and the 

 pharynx is highly susceptible to certain agents but no more susceptible 

 than other parts at the pharyngeal level to others. In general, the poste- 

 rior zooid region is distinctly more susceptible than the posterior part of 

 the anterior zooid (Fig. ^iZ^ -^^D); but under certain conditions — for ex- 

 ample, in neutral or slightly acid cyanide — its relative susceptibility is 

 decreased (Fig. ^t,, E-G), and it may be less susceptible than the anterior 

 zooid. In these respects the cUfferences between anterior and posterior 

 zooids are very similar to those between old and young individuals. 



Susceptibilities of isolated pieces to cyanide differ with length of piece 

 and time after section. Pieces one-third to one-fourth the total body 

 length of animals 1 5 mm. or more in length show little or no increase in sus- 

 ceptibility after section, except immediately adjoining cut surfaces. Simi- 



'* Species used for observation of differential susceptibility are as follows: without pos- 

 terior zooids and fission: Procoiyla flimatilis, Fonticola velata, Phagocata gracilis, Ciirtisia 

 foremami (see p. 41, footnote 7); with posterior zooids and undergoing fission at a definite zone: 

 D. dorotocephala, D. agilis, D. tigrina,a.r\d a Japanese species, externally very similar to Diigesia. 



