46 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



decrease continues to the posterior end (Fig. 22). As these figures also 

 show, the length of regenerated tissue posterior to the eyespots increases 

 from anterior to posterior levels of the anterior zooid and is again less in 

 the posterior zooid region (Fig. 21), or in species without posterior zooid it 

 increases to the posterior end (Fig. 22). In other words, with increasing 

 distance of level from the anterior end, an increasing proportion of the 

 anterior region of the new individual is formed by regeneration rather than 

 by redif^erentiation, with decrease in regeneration again posterior to the 

 fission zone when it is present. These differences in relative size and pro- 

 portion of parts, as primarily localized and determined in pieces from dif- 

 ferent body-levels, gradually approach the normal in consequence of dif- 



P^iG. 22, A-D. — Differences in prepharyngeal length and position of pharynx in pieces from 

 anterior and posterior levels. A, B, Curtisia foremanii; C, D, Fonticola vclata. 



ferential growth. Parts that are "too small" grow more rapidly than other 

 parts in later stages, or in animals kept without food, at the expense of 

 other parts with reduction in size of the whole. Changes in proportion are 

 much more rapid in fed than in starved animals. 



Certain planarian species show increasing inhibition of head develop- 

 ment with decrease in length of piece below a certain fraction of body or 

 zooid length. The length of piece at which inhibition becomes evident in- 

 creases from anterior levels posteriorly or to the fission zone, but in the 

 posterior zooid region it becomes very small. In pieces of equal length be- 

 low a certain fraction of body or zooid length the degrees of inhibition in- 

 crease from anterior to posterior levels, or to the fission zone, and are much 

 less in the posterior zooid.'-' The inhibited head forms and conditions 

 which determine them are discussed in other connections (pp. 177-96). 



The polyclad turbellarian Lcptoplana docs not reconstitute a head 



'3 For data see Child, 191 ib; Child and Watanabe, 1935a; Watanabe, 19356; Sivickis, 1923, 

 i93ici, b, 1933; Abcloos, 1930; Buchanan, 1933. 



