290 BIOLOGY OF THE PROTOZOA 



positive if the organisms are mature (Fig. 137, p. 268). All of 

 these facts, and the literature contains many other similar cases, 

 indicate that environmental stimuli are without effect in producing 

 conjugations unless the protoplasm is in a condition where such 

 conjugations are possible. Indeed, when fully mature, i. e., when 

 the protoplasmic conditions are just right for conjugation, union 

 will take place in a rich food medium and without the transition 

 from full nourishment to hunger. This phenomenon is abundantly 

 illustrated in the records of Uroleptus mobilis and in my records 

 of Paramecium caudatum, Bhpharisma undulans, or of Didinium 

 nasntum. There is little information as to the exact nature of these 

 protoplasmic conditions prior to conjugation. Zweibaum (1922) 

 gives good evidence to show that the quantity of glycogen in the 

 cell is reduced to a minimum at this period, the large drops of 

 neutral fat disappear while small droplets of another type make 

 their appearance together with some cholesterine ester and large 

 quantities of what was interpreted as fatty acids. These are prob- 

 ably effects of inadequate food material, for the observer obtained 

 similar results with Paramecia under conditions of starvation which 

 were not followed by conjugation. 



II. INTERNAL CONDITIONS AT THE PERIOD OF FERTILIZATION. 



In the last analysis both internal and external conditions play 

 their respective parts in protoplasmic preparations for conjugation. 

 Without external stimuli, without oxygen and food, vitality would 

 soon cease; with them, vitality manifested by metabolism and 

 reproduction will continue. With metabolism, however, the pro- 

 toplasmic make up is constantly changing and these changes are 

 shown by the general reactions and by the organization (see Chap- 

 ter V). According to Hertwig (1908), Popoff (1908), and Rautmann 

 (1909), the changes thus brought about lead to disturbances of the 

 normal ratio of nucleus to cytoplasm (Kernplasmaverhaltnis) 

 and lead to conjugations whereby the normal relation of nucleus 

 to cytoplasm is regained. W'hatever the changes due to metabolism 

 are in a given case the conclusion is forced upon us by the mass of 

 evidence that given external conditions will provoke conjugations 

 at one period of the life cycle and will have no effect in producing 

 them at another period, while at the critical period of maturity 

 external conditions may be entirely negligible as they appear to be 

 in the Coccidiomorpha and in gamete-forming organisms generally. 

 Here protoplasmic and not external conditions control the issue. 

 There is some significance in the fact that encystment (with endo- 

 mixis) is induced by the same external conditions as is conjugation. 

 Mengheni (1913) found that Stylonychia will not encyst if food is 

 abundant but that hunger and low temperature are necessary con- 



