3S6 PATTERNS AND PROBLEMS OF DEVELOPMENT 



zation induced by the anterior activation is not specifically different from 

 that induced anterior to a posterior section/'^ In a few annelid species a 

 large, and apparently indefinite, number of segments may regenerate an- 

 teriorly as well as posteriorly, as, for example, in Criodrilus (Janda, 1912a, 

 h; Tirala, 191 2). These differences in anterior reconstitution are similar 

 to those in different planarian species, that is, in some only a head region 

 regenerates, other parts developing by reorganization of levels posterior 

 to the level of section, while in others parts posterior to the head also 

 regenerate and there is little reorganization of old parts. These differences 

 doubtless depend in both groups on physiological conditions determining 

 rate of regeneration and rate or capacity for reorganization. In planarians 

 it has been possible, by means of inhibiting conditions, to bring about 

 head development almost wholly by reorganization with regeneration only 

 at the extreme tip (see Fig. 72, H, /, p. 191). As in planarians, headless 

 annelid pieces can determine development of posterior ends, but nothing 

 anterior to the level of origin of the piece develops except in relation to 

 a developing head region or a region activated by section and isolation 

 from more anterior levels. Some species show great variation in scale in 

 relation to length of piece, complete individuals developing from a single 

 segment in certain forms, in others only from longer pieces.'^ The varia- 

 tion in position of regenerated sex organs in reconstituted Criodrilus 

 (Janda, 1912a, h) suggests variarion in scale determined by differences in 

 dominance of the developing head region. 



Short annehd pieces, including the head, or from levels near the head, 

 often fail to regenerate posterior ends, again like Euplanaria. This has 

 commonly been regarded as indicating lack of potency to develop a pos- 

 terior end in this anterior region, but the experiments on scale of organi- 

 zation in short anterior planarian pieces suggest that in the annelid, as in 

 the planarian, scale of organization may sometimes be "too long" for the 



piece. 



In buds, both plant and animal, the dominant apical region is the first 

 part of the new polar axis to appear. Adventitious plant buds may origi- 

 nate within a single cell or involve a group of cells (see pp. 17-19), but 

 the scale of the bud pattern increases with progress of development, and 

 successive axial levels are progressively determined. Buds of multicellular 

 animals apparently involve a considerable number of cells in their eariiest 

 stages, and considerable increase in size may occur before scale of organi- 



■6 Berrill, 1931; Berrill and Mees, 1936; Gross and Huxley, 1935. 

 '7 Morgulis, 1907; Korschelt, 1919; Dehorne, 1932; Martin, 1933. 



