126 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



posterior end. This was perhaps a species in which new segments are not 

 formed after hatching (Sun and Pratt, 1931). In all forms tested for dye 

 reduction a distinct ventrodorsal reduction gradient appears, rate of re- 

 duction decreasing from the mid- ventral region laterally and dorsally. In 

 the more or less transparent microdrilous oligochetes the segmental gan- 

 glia are visible through the body wall in the living individual. With sufh- 



If^" 



70 90 110 150 



B 



A 



Fig. 42, v4-C.— Susceptibility gradients of microdrilous oligochetes. A, Litmbriculiis in- 

 constans; B, Tubifex rivitlorum; C, Dero limosa. In B and C ordinates indicate minutes; ab- 

 scissae, body-levels as segment number from anterior end at left (from Hyman, 1916a). 



cient exposure to the dye they become stained, and repeated observations 

 seem to show a higher rate of reduction in them than in the ventral body 

 wall. However, since they are seen through the stained body wall, the 

 possibility of error exists. 



Susceptibility gradients observed by Hyman (1916a) in microdrilous 

 oligochetes parallel the reduction gradients. Except in the fully developed 

 Aeolosoma individual, the forms examined show a U-shaped death gradi- 

 ent. Different species differ as regards relative length and height of an- 

 terior and posterior arms of this gradient (Fig. ^2,A,B,C). The anal seg- 



