CERTAIN GENERAL CHARACTERISTICS 43 



certain length the posterior part of the body consists of one or more zooids, 

 the number of zooids increasing with body length.^ Occurrence of a second 

 set of sex organs in the posterior zooid is certainly rare but has been ob- 

 served by Kenk (1935a) and is regarded by him as morphological evidence 

 of the presence of a posterior zooid. Fission can be prevented or induced 

 experim.entally in D. dorotocephala. When it is prevented, animals 30-35 

 mm. in length, sometimes even longer, result. These give evidence of the 

 presence of two, three, or more zooids, and fission can be induced at sev- 

 eral levels (Fig. 16, F, F' , F") of the postpharyngeal region (Child, 1910a). 

 In the species which do not undergo fission, no evidence of any kind indi- 

 cating presence of a posterior zooid has been found. Reconstitution in 



17 18 



Figs. 17-20. — Planarian regeneration. Fig. 17, difference in rate at anterior and posterior 

 surfaces of section and between median and lateral regions of a single surface. Fig. 18, more 

 rapid regeneration ventrally than dorsally. Figs. 19, 20, regeneration in certain concentrations 

 of Ringer solution without contraction of surface of section; cut ends of gut unite with ecto- 

 derm forming an opening into gut anteriorly and posteriorly; greater tolerance to Ringer an- 

 teriorly than posteriorly. 



most planarians consists of more or less regeneration at surfaces of section 

 with redifferentiation elsewhere, but in some it consists chiefly or wholly of 

 regeneration. 



As regards rate of reconstitution, it may be noted first that in isolated 

 pieces, except when very short, regeneration in the earlier stages is more 

 rapid at the anterior, than at the posterior, end of the piece (Fig. 17).'' 

 Since this dift'erence in early stages has no constant relation to the final 

 amount of regeneration, it suggests the existence of a longitudinal differ- 

 ential or gradient in the planarian body. Early growth of new tissue is 



* Child, 1910a, 191 1&, d, 1913&, 1930; Hyman, 19236. 



9 This difference has been observed in D. ligrina and D. agilis (Child, unpublished), in the 

 form called Planaria lata by Sivickis (1923), in a Japanese species resembling D. dorotocephala 

 (Child, 19326), in Phagocata gracilis (Buchanan, 1933), and repeatedly by groups of students in 

 the study of reconstitution of D. dorotocephala. 



