322 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



Fig. 112. — Elonga- 

 tion and permanent 

 alteration of form in 

 Dugesia doroloceph- 

 ala resulting from re- 

 peated independent 

 reaction of posterior 

 zooid without fission. 



The act of fission in large intact animals may begin 

 with independent motor reaction of the posterior zooid 

 region, consisting in attachment to the substratum, while 

 the anterior zooid attempts to advance. In the struggle 

 the body anterior to the fission zone often becomes 

 greatly elongated until rupture results. Following rup- 

 ture, both zooids undergo reconstitution. Fission some- 

 times results from sudden longitudinal contraction an- 

 terior and posterior to the fission zone. With slow growth 

 in length in the laboratory animals may attain more than 

 twice the usual length without fission; in these growth 

 beyond a certain length is almost entirely in the posterior 

 zooid region (see Fig. i6, p. 42). In many of them the 

 posterior region shows independent reaction; but fission 

 is not completed, either because the animals are not ac- 

 tive enough or because the tissues rupture less readily 

 than in smaller, younger animals. By frequent repetition 

 of the independent reactions the posterior part of the 

 anterior zooid becomes permanently much elongated and 

 very slender (Fig. 112), and there is complete or almost 

 complete absence of control of the posterior zooid region 

 by more anterior levels. It is merely dragged about as the 

 anterior zooid advances and often interferes with the ad- 

 vance by attaching itself, but it is usually detached sooner 

 or later by the struggles of the anterior zooid or may re- 

 lease itself after the anterior zooid has ceased to struggle. 

 In these animals physiological isolation of the posterior 

 zooid has become directly visible. 



Fission is readily induced by removal of the head, even 

 in animals far below the length at which it usually occurs 

 (Child, 1910a). Low concentrations of anesthetics and 

 other inhibiting agents also induce fission, apparently 

 by decreasing dominance; and a second division of the 

 posterior fission piece often follows the first. Under these 

 conditions fissions are usually limited to the period when 

 the regenerating head has developed far enough to deter- 

 mine more or less motor activity but apparently has not 

 attained full dominance. If there is no fission during this 

 period, it usually occurs only after increase in length or 

 another removal of the head. When animals with differ- 



