140 



W. A. CASTLE. 



under favorable conditions develop after the manner of those of 

 other planarians ; however sufficient material has not yet been 

 available to make possible a study of the embryonic stages. 



Following- sexual maturity or with the advent of unfavorable 

 conditions before sexual differentiation is complete, it has been 

 observed that the worms undergo a histological degeneration 

 similar to that which precedes the fragmentation of an asexual 

 stock. This may at first involve only the structures associated 

 with the reproductive system. In this event the worms return to 

 the asexual state and a period of growth may ensue before the 

 ultimate degeneration that leads to fragmentation and encystment. 

 Under conditions that are more decidedly adverse to sexuality a 

 degeneration of the other internal structures may accompany 

 that of the sex organs, and fragmentation and encystment will 

 follow without any intermediate period of asexuality and growth. 



It has been found that the life cycle of Planaria velata may be 

 altered and controlled by subjection to the proper experimental 

 conditions. This is true not only of the asexual cycle but of the 

 sexual cycle as well. In nature the conditions bringing about en- 

 cystment are a period of good nutrition and growth accom- 

 panied by an increase in the temperature of the water. Child 

 observed that by partial starvation the onset of fragmentation and 

 encystment might be warded off indefinitely. Worms were thus 

 maintained at approximately the same size and in approximately 

 the same physiological condition for more than two years 

 (Child, '14). 



Keeping the worms at a lower temperature has been found to 

 prolong the period of growth that precedes encystment but not 

 to inhibit encystment for an indefinite period of time. At room 

 temperatures (18-20 C.) worms will cease feeding and proceed 

 towards encystment when they have attained a length of twelve 

 or fourteen millimeters, whereas worms kept in an ice box at 

 8-10 will attain a length of eighteen or twenty millimeters be- 

 fore the onset of fragmentation. Eventual encystment however 

 is the result always to be expected when the temperature remains 

 constant. Conversely it is found that at temperatures higher 

 than normal worms can be induced to encyst at a much smaller 

 size. The approach of encystment can be detected in advance by 



