144 



W. A. CASTLE. 



but have shown no development of sexual structures. It is ap- 

 parent from the results of these experiments that temperature is 

 an important factor in the control of sexuality in Planaria velata. 

 From observations of the conditions under which sexuality is 

 known to occur in nature it is probable that the rate of growth is 

 also important. Experiments along these lines are being con- 

 tinued. 



Histological studies have been made upon the differentiation of 

 the sexual structures as well as upon the anatomy of the reproduc- 

 tive system of sexually mature worms. Other studies have been 

 made upon the degeneration of sexual structures and the changes 

 that accompany and precede either fragmentation or a return to 

 the asexual condition. The structure of encysted fragments and 

 the process of reconstitution within the cyst have been made the 

 subject of further histological investigation. The encysted frag- 

 ments consist of a layer of epithelial cells forming a sac which en- 

 closes a mass consisting largely of globules of fats, proteins, etc., 

 but containing a few scattered nuclei. Some degree of proto- 

 plasmic continuity evidently exists throughout this mass, for in 

 the process of reconstitution the pharynx arises from the middle 

 of the mass at a considerable distance from any part of the wall. 

 The region of the developing pharynx first becomes distinguish- 

 able as an area containing rapidly increasing numbers of nuclei. 

 It has not been determined whether the axis of the new worm is a 

 result of the gradient present in the old worm which has per- 

 sisted through the processes of fragmentation and encystment or 

 the result of a new gradient that has arisen because of conditions 

 to which the cyst is subjected. 



LITERATURE CITED. 



Alexander, M. I., and Price, H. F. 



'26 Histological Degeneration Preceding Fragmentation and Encyst- 

 ment in Planaria velata. Report of meeting of Am. Soc. of Zool. 

 Anat. Rec., XXX I V. 

 Child, C. M. 



'13 The Asexual Cycle in Planaria velata in Relation to Senescence and 

 Rejuvenescence. BIOL. BULL., XXV. 



'14 Asexual Breeding and Prevention of Senescence in Planaria rclata. 



BIOL. BULL., XXVI. 

 Stringer, C. E. 



'09 Notes on Nebraska Turbellaria, with Description of Two New 

 Species. Zool. Anz., XXXIV. 



