AGAMIC REPRODUCTION AND REJUVENESCENCE 129 



same physiological condition, but undivided, was kept without 

 food during the same period as a control. In Fig. 35, curve ab 

 shows the susceptibility of the anterior fission-pieces, curve cd 

 that of ten of the undivided animals, also without food. At this 

 time the animals had attained the stage of development shown in 



Fig. 34- 



The susceptibility of the fission-pieces is distinctly greater 

 than that of the undivided animals, and as a matter of fact the 

 differences are greater than the curves show. At the points in the 

 curve where the two lots appear to be in the same or nearly the 

 same stage of disintegration, examination of the pieces showed 

 that even though the two lots might fall within the same one of 

 the five arbitrarily distinguished stages, the fission-pieces were 

 always more advanced in that stage. The fission-pieces are evi- 

 dently younger physiologically than whole worms, and this is true, 

 not only for the posterior region where the reconstitutional changes 

 are localized, but for the whole body, including the head. Un- 

 doubtedly the anterior regions have served to some slight extent 

 as a source of energy for the developmental changes in the posterior 

 region. 



Similar results have been obtained repeatedly in other similar 

 experiments. If the anterior fission-pieces are fed during recon- 

 stitution and their susceptibility compared with that of whole 

 animals fed at the same time, the increase in susceptibility is found 

 to be less marked or inappreciable. In such cases the food taken, 

 rather than the tissues, provides the energy for the development 

 of the new posterior end. Similarly the larger the animal when 

 division occurs, the less the increase in susceptibility. In the very 

 large, heavily fed animals, in which the anterior fission-piece may 

 be fifteen millimeters or more in length, there is usually no appre- 

 ciable increase in susceptibility in this piece after fission. Here 

 the amount of reconstitutional change is so slight in relation to 

 the size, and the amount of nutritive reserve is so great, that the 

 body as a whole is not appreciably affected by the development 

 of the posterior end. 



The relation between agamic reproduction and susceptibility 

 is the same in Planaria dorotocephala and in P. maculata. In both 



