ASEXUAL CYCLE OF PLAN ARIA VELATA. 2OI 



substances within the cell brings about a decrease in the general 

 metabolic activity per unit of weight or volume because it de- 

 creases the proportion of the material involved in the general 

 metabolic reactions to the inactive or less active material. 

 The decrease in the proportion of the general metabolic substra- 

 tum characteristic of the embryonic cell constitutes to some 

 extent a histological criterion of the physiological change in the 

 cell. 



In short, the decrease in rate of metabolism per unit of sub- 

 stance, which is characteristic of development and senescence, 

 is undoubtedly due in part to the fact that the proportion of the 

 cell substance concerned in the general metabolic activity is 

 decreasing and the proportion of less active or relatively stable 

 substance is increasing. Changes in the size of the cell or in the 

 size relations of nucleus and cytoplasm (Minot, '08) are not 

 necessary factors in the result. 



To what extent the decrease in the rate of metabolism during 

 senescence is due in a given case to actual decrease in the rate 

 of chemical reaction and how far to a decrease in the proportional 

 amount of chemically active or more active substance is often 

 difficult to determine, but it is probable that in some cases, or 

 ven in some cells of the individual, the one factor and in others 

 the other is the more important. 



As regards Pla naria velata, the facts are that the rate of metab- 

 olism decreases during growth and development and increases 

 when the substances previously accumulated are removed, either 

 by regulatory reorganization, or by starvation. These facts 

 show very clearly that in one way or another the accumulation 

 of material in development decreases the rate of metabolism and 

 its removal brings about an increase in rate. Senescence and 

 rejuvenescence in this species consist essentially, I believe, in 

 these changes. 



SUMMARY. 



i. After a period of growth and activity Planaria velata under- 

 goes fragmentation from the posterior end forward, the frag- 

 ments encyst and give rise by a process of regulation to whole 

 worms of small size. 



