368 PATTERNS AND PROBLEMS OF DEVELOPMENT 



new gradients and induction of reorganization, annelid, and planarian 

 reconstitution do not differ greatly. It was noted earlier that in most 

 annelids only a certain number of segments, characteristic for the species, 

 regenerates anteriorly after removal of a larger number, the other remain- 

 ing segments removed being formed by reorganization of old segments 

 induced by the new dominant region. A few annelids regenerate a large 

 number of segments anteriorly.^ Attention is called again to the case of 

 Sahella in which "abdominal" and ''thoracic" segments differ morpho- 

 logically. Regeneration from anterior ends at abdominal levels induces 

 reorganization of some abdominal into thoracic segments. This reorgani- 

 zation may also occur after section and before head regeneration. Also, 

 in some cases reorganization of segments anterior to a posterior section in 

 the abdominal region takes place ; this represents an approach to bipolar- 

 ity. Apparently the activation following section and isolation is sufhcient 

 to bring about induction without actual head regeneration.^ According 

 to Berrill and Mees, visible light increases greatly the number of segments 

 reorganized; in some way it apparently makes the dominance established 

 following section effective over a greater distance. The authors suggest 

 that the effect of light involves a photochemical reaction and release of 

 electrical energy rather than diffusion of a chertiical substance. 



Regeneration of posterior ends in planarians and anneUds involves es- 

 tablishment of a growth gracUent opposite in direction to the primary 

 polar gradient. In short planarian pieces and apparently also in Lumhricu- 

 lus the posterior activation may inhibit head regeneration (pp. 183, 406). 

 After completion of posterior regeneration in planarians this gradient per- 

 sists chiefly at the extreme posterior end and is associated with growth in 

 length; but in the species with one or more posterior zooids these grow 

 in length more rapidly than the anterior zooid. In the annelids the pos- 

 terior gradient becomes a gradient of segment development and growth, 

 with high end posterior, but anterior to the anal segment, and may per- 

 sist throughout life in some species or disappear later in others. 



Bipolar partial forms with heads at both ends develop in pieces from 

 anterior regions, and bipolar posterior forms in pieces from posterior re- 



' For numbers of segments regenerated anteriorly and reorganization of old segments see 

 Ivanov, 1908; Hyman, 19160, and citations; also Morgulis, 1907; Korschelt, 1919; Martin, 

 1933; also Hyman, 1940, "Aspects of regeneration in annelids," Amer Nat., 74. 



8 Berrill, 1931; Berrill and Mees, 1936a, h; Gross and Huxley, 1935. In his earlier paper 

 Berrill regards the regenerated head as organizer, but it seems evident from the later papers 

 that the activation following section, rather than the head itself, is the inducing factor. 



