54 J- MCA - KATER AND R. D. BURROUGHS. 



A I on the sixtieth day we must suppose, if it is a matter of 

 selection, that the non-encysting variations are more viable than 

 the encysting. This does not seem reasonable, because if it 

 were so we would hardly expect any existing Polytomella to 

 exhibit the phenomenon. 



No attempt has been made to accurately measure the division 

 rate in a non-encysting strain, such as F2, as compared to an 

 encysting, F^. Mass observations lead us to believe that even 

 when Polytomella has reached a condition of morphological 

 degeneracy and may have entirely lost the power to encyst when 

 they are transferred to a fresh and suitable medium, they are 

 stimulated to a perfectly normal period of growth and division, 

 with the exception that it does not end in encystment. 



The morphological degeneration observed in the cultures Ai, 

 Bi, D, E, etc., is very similar to that noted by Maupas in Sty- 

 lonychia when conjugation was prevented and by Calkins in 

 Uroleptus. We will not enter into the propriety of terming this 

 senescence, and encystment as a rejuvenating process. 



SUMMARY. 



1. Polytomella citri encysts only when the cell-body contains 

 considerable starch. 



2. Encystment is not due to any perceptibly adverse environ- 

 mental factors. 



3. Optimum conditions for growth and reproduction are con- 

 comitant w^ith maximum encystment. 



4. Prevention of encystment, either by continuous transfer or 

 by low temperature, if carried to sufficient extent, will result 

 in morphological degeneracy and loss of the tendency to store 

 starch and to encyst. 



LITERATURE. 

 Allman. 



'72 On Some Points in the Development of Vorticellidae. Quart. Jour. Micr. 



Science, ad series, 12. 

 Auerbach, L. 



'54 Ueber Encystierung von Oxytricha pellionclla. Zeitsch. f. \viss Zool., 5. 

 Brand, Th. 



'23 Die Encystierung bei Vorticella microstoma nml hypotrichen Infusorien. 



Arch. f. Protist., 47. 

 Calkins, G. N. 



'09 Protozoology. 



'15 Didinium nasutum. i. The Life History. Jour. Exp. Zool., 19. 



'16 General Biology of the Protozoan Life Cycle. American Naturalist, 50. 



