6 LIBBIE H. HYMAN AND ALBERT E. GALIGHER 



at a more or less definite level near the posterior end (see line x in 

 fig. 2) . When stimulated or placed under altered conditions, the 

 worms are likely to snap ofT their posterior ends, which thereupon 

 regenerate into complete individuals. The vast majority of 

 worms taken in the field will be found regenerating their posterior 

 ends. The place at which the fission occurs is in many indivi- 

 duals detectable by its increased susceptibility to toxic solutions. 

 Whether it preexists before the worm is placed in the toxic solu- 

 tion or whether autotomy is initiated (but not completed or 

 rendered visible) as a consequence of placing the worm in the 

 solution is dfficult to say. Regeneration experiments reported in 

 the paper already referred to indicate that the region preexists. 

 It should be emphasized that not all individuals give evidence of 

 the existence of a breaking level. 



In figure 1 are presented graphs of the death gradients of three 

 individuals of Lumbriculus inconstans in solutions of potassium 

 cyanide. In constructing such graphs the number of segments 

 ■ is plotted on the abscissa and the time of death on the ordinate. 

 As the posterior segments are considerably shorter than the 

 anterior ones the number of segments per unit of the cross-section 

 paper has been increased at the posterior end. The anterior end 

 of the animal is at the left, posterior end at the right. 



The graphs illustrate the general character of the gradient in 

 this species. This type of gradient is characteristic of annelids 

 in general, of vertebrate embryos, and in all probability of all 

 segmented animals, at least in embryonic stages. It is due to 

 the method of growth of segmented animals by the formation of 

 new segments in front of the anal segment. All three of the 

 individuals show the presence of the zone of autotomy described 

 above; in two of them this zone is well marked, while in the 

 third it is only slightly in evidence. 



This susceptibility gradient involves not only the body wall, 

 but also the digestive tract and all visible systems. The trans- 

 parency of this species enables one to see everything except the 

 nephridia. Sex organs are completely wanting. If, then, the 

 susceptibility gradient is in reality a metabolic gradient, it must 

 follow that in this worm pieces cut from the posterior end must 



