PROCESS OF REGENERATION 



121 



that in the early stages the animal pole shows the highest sus- 

 ceptibilit}', but that later a second region of high susceptibility 

 appears in the somatic plate from which the three larval segments 

 developed. Among the microdrilus oligochaetes, I have been 

 able to obtain young of Tubifiex tubifex only. In the spring of 

 1914, nearly all of the individuals of a Tubifex culture which 

 had been kept in the laboratory since the preceding autumn 

 became sexually mature and produced a large number of egg 

 capsules. From these capsules, embryos were removed during 



Fig. 14 Graph of the axial gradient of Tubifex rivulorum, showing the great 

 extent of the secondary rise. Time in hours. 



the later stages of development, and examined in cyanide. It 

 was found that in the stage when the embryo has begun to elon- 

 gate, its posterior region is most susceptible to cyanide, and the 

 susceptibility decreases anteriorly. Four stages in the disinte- 

 gration of an embryo at this stage are shown in figure 16. Later, 

 the head becomes a second region of high suceptibility (fig. 17), 

 and the susceptibility of the head increases until it surpasses 

 that of the posterior end. At hatching, therefore, the head is 

 the most susceptible region. The young worms just after 

 hatching consist of about thirty segments. When placed in 



