14 C. M. CHILD 



numbers, and, by feeding and proper care, development can be 

 carried still farther. 



The susceptibility relations in a stage of six segments differ in 

 certain respects from those in the three-segmented trochophore. 

 The posterior growing region and the head are still the most 

 susceptible regions, the latter being usually slightly less suscep- 

 tible than the former, and in the body region the susceptibility^ 

 decreases anteriorly from the sixth to the fourth or third segment. 

 In the first three or four segments, however, the relations have 

 undergone change. Of these the first segment is most, the 

 second less and the third and sometimes the fourth still less sus- 

 ceptible, just the reverse of the relations in the early three-seg- 

 mented trochophore, but even in the first segment the suscepti- 

 bility is lower than in the head. 



In the six-segmented worm then two opposed susceptibility 

 gradients appear more or less clearly in the ectoderm, and I 

 think also in the mesoderm, for instance, death progresses pos- 

 teriorly from the head' region through the first three or four 

 segments and anteriorly from the posterior growing region through 

 the segments developed later. The possible significance of the 

 changes in these segments and the two opposed gradients in the 

 body will be considered below in the general section (p. 35). 



As in Nereis and other forms, the susceptibility in general 

 increases during the earlier stages of development as the develop- 

 ing animal becomes physiologically younger, but in Arenicola it 

 continues to increase up to or slightly beyond metamorphosis. 

 How much of this latter increase is due to actual intrinsic physio- 

 logical rejuvenescence and how much to increased sensory, 

 muscular and other special functional activity, the susceptibility 

 method does not tell us, but I am inclined to believe that the 

 special functional factors are largely concerned in the increase 

 in susceptibility during and after metamorphosis, and that in 

 this way the earliest stages of senescence are masked and, with 

 this method, become visible only later. For the determination 

 of its intrinsic physiological condition the cell should be isolated 

 from sources of stimulation and this, of course, is less the case 

 in later than in earlier stages of development. 



