2O2 C. M. CHILD. 



2. During the period of growth the worms are undergoing 

 senescence, as the decrease in rate of metabolism indicates, but 

 the small worms which emerge from the cysts are physiologically, 

 as well as morphologically young, possess a high rate of metab- 

 olism and are capable of repeating the life cycle. 



3. In pieces isolated by section the frequency of encystment 

 increases as the level of the piece becomes more posterior in the 

 body, with decreasing size of the piece, with rising temperature 

 and with increasing age of the animal. The facts indicate that 

 encystment is the result of stimulation. The stimulation may 

 result from section, from fragmentation, from a rise in tempera- 

 ture or from other conditions. 



4. The development of the encysted piece into a new whole 

 animal is essentially the same process as the regulatory develop- 

 ment of unencysted pieces. 



5. This species is able to live for an indefinite number of gen- 

 erations without sexual reproduction. Each new asexual genera- 

 tion represents a return to essentially the same physiological 

 and morphological stage. In other words, senescence leads to 

 reproduction and the process of rejuvenescence in each asexual 

 cycle carries the organism back to the same stage of youth. 



HULL ZOOLOGICAL LABORATORY, 

 UNIVERSITY OF CHICAGO. 



REFERENCES. 

 Child, C. M. 



'ua Die physiologische Isolation von Teilen des Organismiis. Vortrage und 

 Aufs. ii. Entwickelungsmech., H. XL 



'lib A Study of Senescence and Rejuvenescence, Based on Experiments with 

 Planarians. Arch. f. Entwickelungsmechanik, Bd. XXXI. , H. 4. 



'lie Studies on the Dynamics of Morphogenesis and Inheritance in Experi- 

 mental Reproduction. I. The Axial Gradient in Planaria dorotoce phala 

 as a Limiting Factor in Regulation. Journ. Exp. Zool., Vol. X., No. 3. 



'lid Studies, etc., II. Physiological Dominance of Anterior over Posterior 

 Regions in the Regulation of Planaria dorotoce phala. Journ. Exp. Zool., 

 Vol. XL, No. 3- 



'ne Studies, etc., III. The Formation of New Zooids in Planaria and other 

 Forms. Journ. Exp. Zool., Vol. XL, No. 3. 



'12 Studies, etc., IV. Certain Dynamic Factors in the Regulatory Morpho- 

 genesis of Planaria dorotoce phala in Relation to the Axial Gradient. 

 Journ. Exp. Zool., Vol. XII., No. i. 



'133 Studies, etc., V. The Relation between Resistance to Depressing Agents 

 and Rate of Reaction in Planaria dorotoce phala and its Value as a Method 

 of Investigation. Journ. Exp. Zool., Vol. XIV., No. 2, 1913- 



