MICHIGAN ACADEMY Ol SCIENCK. 75 



plasm invested itself with a resistent sliell, structureless, but similar in 

 composition to the normal shell. With this preparation, the organism 

 is apparently in a condition to withstand desiccation, freezing, etc., for 

 long continued periods. After tlie expiration of minimal length of time, 

 determined by internal factors, upon finding a suitable environment, the 

 shell is resorbed, and the animal resumes sexual activity. 



In my cultures I have at present (Feb. '17) many cysts, some like 

 those described, but more in which the animal goes into dormant condi- 

 tion without marked bodily changes. The protoplasm is withdrawn into 

 a ring in the dome of the shell, with the two nuclei plainly visible, but 

 somewhat modified. There is no pseudopodial action evident, nor are the 

 contractile vacuoles visible. A membrane across the mouth has often 

 been mentioned under similar circumstances, but I have been unable to 

 find any. Occasionally some of the encysted animals become active, but 

 I have been unable to initiate a renewal of activity by changes in the 

 culture medium, temperature, liglit and other factors. 



SPECIAL BIBLIOGRAPHY. 



1913. Fermor, Xenia. Die Entwickelungsgeschichte von Arcella vul- 

 garis Ehgb. Archiv fiir Protistenkunde. Band 31. 



1910. Khainsky, A. Untersuchungen fiber Arcellen. Arch. f. Protist. 

 Bd. 21. 



1908. Swarczewsky, Boris. Uber die Fortpflanzungserscheinungen 

 bei A. vulg. Arch. f. Protist. Bd. 12. 



Elpatiewsky, W. Zur FortpHanzungen von A. vulg. Arch. f. Protist. 

 Bd. 10. 



Distaso, Arcangelo. Sui processi vegativi e sull'incistidamento di 

 Actinophrys sol. Arch. f. Protist. Bd. 12. 



1907. Awerinzew, S. Beitrage zur Kenntnis der Siisswasserrhizo- 

 poden. Arch. f. Protist. Bd. 8. 



Ibid. Die Struktur und die chemische Zusammensetzung der Geluluse 

 bei der Siisswasserrhizopoden. Arch. f. Protist. Bd. 8. 



1906. Cushman and Henderson. Prel. Study of the finer Structure 

 of A. vulg. American Naturalist. Vol. 40. 



1905. INIartini, E. Beobachtungcn an A. vulg. Zeitschrift fiir 

 wissenschaftliche Zoologie. Bd. 79. 



1898. Rhumbler, L. Zelleib-, Schalen-, und Kernverschmelzung bei 

 Rhizopoden. Biol. Centralblatt. Bd. 18. 



1896. Ibid. Beitrage zur Kenntnis der Rliizopoden. Zeitschrift f. 

 wiss. Zool. Bd. 61. 



Delage et Herourd. Trait(^ de Zoologie Concrete. 



