AGAMIC REPRODUCTION AND REJUVENESCENCE 135 



i.i. and i . 2 . , but zooid 2 has not yet divided. In Fig. 39 zooid 

 i.i. has divided again into i . i . i. and 1.1.2., zooid i . 2. has not 

 yet divided, and zooid 2. has divided into 2.1. and 2.2. In Fig. -39 

 still further divisions have occurred, and the relations of the differ- 

 ent zooids are indicated by the numbers designating each. Here 

 morphological development of each zooid is almost completed 

 before separation occurs. The first separation takes place at the 

 most advanced fission-plane and as other zooids reach a correspond- 

 ing stage other separations occur, but meanwhile new zooids have 

 begun to develop. Thus the breaking up of the old chains and the 

 formation of new go hand in hand. 



Such processes of agamic reproduction do not differ essentially 

 in any way from the process of reconstitution of pieces isolated by 

 section in the same species. In both cases a certain region of the 

 body gradually transforms itself into a whole animal. In both 

 cases certain parts atrophy and disappear, cell division and localized 

 growth occur, and new parts develop. In Stenostomum, however, 

 the new zooid receives food during its development, for the ali- 

 mentary tract common to the whole chain passes through it; con- 

 sequently it is not dependent upon its own tissues for the energy 

 necessary for its development as is a physically isolated piece, and 

 therefore it does not undergo the reduction in size characteristic 

 of such pieces. In fact it usually increases in size during develop- 

 ment. 



In Stenostomum as in Planaria the susceptibility method 

 demonstrates the existence of a longitudinal axial gradient in rate 

 of metabolism. Before agamic reproduction begins this gradient 

 extends the length of the individual, but as new zooids arise the 

 anterior region of each shows a higher rate of metabolism than the 

 region immediately anterior to it, and each zooid develops its own 

 axial gradient like that of the original animal. In the earlier stages 

 of zooid development the susceptibility of the new zooid is less, 

 i.e., its rate of metabolism is lower, than that of the fully developed 

 zooid which heads the chain, but as development proceeds the sus- 

 ceptibility increases, until at the time of separation, or soon after, 

 it is higher than that of the anterior zooid. Separation of the 

 zooids at an earlier stage of development than that at which it 



