124 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



length must be made; effects of conditions favoring or stimulating motor 

 activity and those favoring quiescence must be evaluated as far as possi- 

 ble before reasonable certainty can be attained as to the presence or ab- 

 sence and significance of a respiratory gradient independent of motor ac- 

 tivity and other incidental factors. At present most of the evidence indi- 

 cates presence of a U-shaped 



_\ L respiratory gradient and is in 



agreement with results of dye 

 reduction, susceptibiHty, elec- 

 tric-potential difference, and 

 galvanotactic reactions, some 

 of which are obtained in com- 

 plete absence of motor ac- 

 tivity. 



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\ 



1 



Observations on differential 

 dye reduction in several micro- 

 drilous oligochete species and 

 several earthworms show a 

 U-shaped reduction gradient 

 with posterior arm much long- 

 er than anterior, except in cer- 

 tain forms after segment de- 

 velopment is completed and 

 the posterior growing region 

 disappears. In zooid chains of 

 Nais and Aeolosoma each new 

 zooid develops a reduction 

 gradient with rate decreasing 

 posteriorly from the head re- 

 gion and anteriorly from the 

 posterior-segment-forming re- 

 gion, when that is present and 

 active. In Aeolosoma the posterior arm of the gradient is present only 

 temporarily, since the posterior growing region disappears after a certain 

 number of segments is formed, and the gradient in the fully developed in- 

 dividual becomes a simple gradient, decreasing from the head region to 

 the posterior end (Child, 1934a; Child and Rulon, 1936). 



Stages of differential dye reduction in Tuhifex are indicated diagram- 

 matically in Figure 40. Reduction is most rapid anteriorly and posterior- 



FiG. 40. — Diagrammatic, indicating successive 

 stages of differential reduction of dyes in body wall 

 of Tubifex tuhifex when not appreciably injured by 

 the dye. Ordinates indicate degree of reduction ; hor- 

 izontal base line, no visible reduction; anterior end 

 at left; diagram at bottom indicating reduction in 

 animal with well-developed clitellum, C (from Child 

 and Rulon, 1936). 



