172 SENESCENCE AND REJUVENESCENCE 



disintegration begins and the whole animal, including the head, 

 dies. In the rounded mass the internal structure gradually dis- 

 appears with extensive necrosis and disintegration of cells, until 

 little more than a sack remains containing some living tissue and a 

 large amount of granular substance resulting from the cell disinte- 

 gration. In other words, this mass represents to a large extent a 

 process of involution and death of cells and tissue. 



In those individuals in which this process of involution ceases 

 at the stage of Fig. 59 or Fig. 60, the mass usually does not undergo 

 complete disintegration, but remains attached to the body and is 

 gradually resorbed, the process extending over a month or two. 

 During this time the mass evidently serves as a source of nutrition 

 for the active region and is in some sense analogous to the yolk 

 sac of many embryos. In such individuals the anterior region 

 remains continuously active and the involution mass gradually 

 becomes smaller (Figs. 62 and 63), until completely resorbed and 

 only a longer or shorter anterior region considerably reduced in 

 size remains. In cases where the resorption of the posterior mass 

 begins at a stage like that of Fig. 59, the portion of the body remain- 

 ing after complete resorption may include the anterior half, but 

 where resorption does not begin until involution is more advanced, 

 as in Fig. 60, the portion remaining after resorption may be only 

 the anterior fourth (Fig. 64). 



After resorption of the posterior mass is completed, the remain- 

 ing portion slowly undergoes reconstitution, developing a new 

 posterior end and a new pharynx and mouth (Fig. 65), and thus 

 finally attaining the same condition as in those cases where the 

 involution mass disintegrates and is lost without resorption. At 

 this stage the small animal is physiologically young, as its high 

 susceptibility indicates, and is again ready to take food and grow 

 and repeat the life cycle. 



In this remarkable process of senescence and death of a part of 

 the body and rejuvenescence of the remainder, no reproductive 

 process is involved except the reconstitution of the anterior region 

 into a new whole. That portion of the body which under natural 

 conditions undergoes fragmentation and encystment, the fragments 

 undergoing reconstitution to new animals, is in these cases appar- 



