PHYSIOLOGICAL CHARACTERISTICS OF AXIATE PATTERNS 125 



Fig. 41. — Stages of differential dye reduction in two- 

 zooid chains of A eolosoma, Nais, and Stylaria, indicated 

 as in Fig. 41 ; fission zone at F (from Child and Rulon, 

 1936). 



ly, except that the anal segment, indicated by the short heavy hne on the 

 base line at the right, reduces slowly (Fig. 40, top). The anal segment, of 

 course, is not a part of the posterior growing region but is the original pos- 

 terior end of the body. It is interesting to find it so sharply marked off 

 from the growing region immediately anterior to it. 



A stage of reduction in animals with well-developed clitellum (C) is in- 

 dicated in the diagram at the bottom of Figure 40. The clitellum is sharp- 

 ly distinguishable from ad- 

 joining regions by its much 

 higher rate of reduction. 

 With overstaining, differen- 

 tial susceptibility to the dye 

 results in earlier injury to 

 anterior and posterior re- 

 gions than to other parts, 

 with U-shaped gradient. Re- 

 duction is differentially re- 

 tarded in the injured regions, with the result that the normal reduction 

 gradient undergoes complete inversion, the middle region reducing most 

 rapidly, though no more rapidly than in normal animals, and rate of re- 

 duction decreasing both anteriorly and posteriorly. 



The normal U-shaped gradient appears in actively motile animals and 

 also in animals with motility practically completely eliminated by anes- 

 thesia, though possibly slightly less steep in the anesthetized animals. 

 Motor activity of animals isolated for a week or two in clear water in day- 

 light decreases greatly. These animals often show no visible movement 

 when observed continuously during the period of dye reduction, but the 

 reduction gradient is essentially the same as in actively motile animals. 

 Evidently differential dye reduction does not result from differential 

 motor activity. Differential dye reduction in two-zooid chains of A eolo- 

 soma, Nais, and Stylaria is indicated in Figure 41, the fission plane being 

 at F. This gradient may also be completely inverted by overstaining and 

 differential injury of anterior and posterior regions of each zooid. Motor 

 activity is certainly not greater in the region of fission than in adjoining 

 regions; it is probably less, for the posterior region of the anterior zooid 

 and the head of the second zooid are not fully developed. 



In several unidentified species of earthworms a similar two-armed re- 

 duction gradient is present. In one form, however, without indication of 

 loss of posterior segments, rate of reduction decreased from anterior to 



