C. DOBBLL 167 



the production of females \ In all his experiments, however, starvation 

 introduces a complication : so that after carefully studying the facts 

 which he, Prandtl, and Hertwig have so far recorded, I am still unable 

 to understand the real significance of these interesting observations. 

 They seem to me suggestive rather than demonstrative. 



96. The method employed by Maupas (1889) for inducing ciliates 

 to conjugate consisted in the simple procedure of transferring some 

 individuals from a large stock culture to a small culture on a slide. 

 As soon as the food in the smaller culture was exhausted, conjugation 

 occurred — provided the animals were " sexually mature." Essentially 

 the same measure has been frequently used with success by other 

 workers (Hamburger, 1904 ; Calkins and Cull, 1907 ; Enriques, 1907 ; 

 Jennings, 1913; Calkins and Gregory, 1913; Woodruff, 1914; etc.). 

 Jennings (1913) describing his method says: "In the evening large 

 numbers of the animals [P. caudatum and P. cmrelia] were taken from 

 the large cultures and placed in watch glasses; early the following 

 morning they were usually beginning conjugation." He believes that 

 conjugation occurs " not as a result of starvation, but at the beginning 

 of a decline in the nutritive conditions, after a period of exceptional 

 richness that has induced rapid multiplication" (Jennings, 1910). Calkins 

 had earlier (1902) concluded that "hunger is not a pre-requisite for 

 union, it apparently prevents conjugation." 



97. Enriques has long maintained that " the necessary and sufficient 

 conditions for conjugation are environmental conditions" (1909 a). He 

 states (1907) that Colpoda steini will conjugate only when the depth of 

 the culture is 2 — 3 mm.- — never in deeper cultures. He further states 

 that the liquid from a culture in which conjugation is taking place, 

 when added to a non-conjugating culture, brings about conjugations in 

 it : and reversely, liquid from a non-conjugating culture, added to a 

 conjugating culture, causes cessation of conjugation. He concludes 

 that the onset of conjugation "does not depend upon mysterious con- 

 ditions developing themselves in the infusoria, but the modifications of 

 the circumambient liquid play the chief part." 



98. Pursuant of this train of thought, Enriques (1909) has per- 

 formed a number of experiments with Gryptochilum nigricans. He 



' Popoff incorrectly (§ 14) calls males aud females " microgametes " and "macro- 

 gametes." 



^ Compare in this connexion the much earlier experiments of Everts (1873) on 

 Vorticella. He believed that conjugation was caused by the drying up of the water 

 iiu which the animals lived. 



