CHAPTER VIII 

 INTERNAL CHARACTERISTICS AND SIZE 



1. Assortative Mating 



Conjugation, signalized by fusion and nuclear ex- 

 change, occurs in a great many unicellular spe- 

 cies; but only in ciliates the act of mating has been 

 studied in relation to body size. 



Conjugants are smaller. In some species, conjugat- 

 ing individuals are characteristically smaller than non- 

 conjugants. Maupas ('88) measured the lengths of 

 both kinds; in Paramecium aurelia conjugants were 75 

 to 145 micra long, while non-conjugants of the same 

 cultures were 125 to 200 micra long. Similar smaller 

 sizes were found in the conjugants of other species; 

 while in a number of species, as Paramecium bursaria 

 and Colpidium (Maupas, '89), the sizes of conjugants 

 did not differ from those of the rest of the population. 

 Pearl ('07) found that in a general population of Para- 

 mecium caudatum the inferiority of size amounted on 

 the average to 15 per cent in length, and 18 per cent 

 in breadth; therefore to about 43 per cent in volume. 

 But within a clone the difference was somewhat smal- 

 ler (Jennings, '11), amounting to 8.5 percent in length; 

 which is nevertheless a highly significant difference. 



This smaller size might represent an actual reduc- 

 tion of body volume through the expulsion of water, 

 perhaps due to changes in the physical and chemical 

 characters of the body substances in preparation for 

 conjugation. Zweibaum ('22) found an increase of 

 about 300 per cent in oxygen consumption per Para- 

 mecium individual at the initiation of conjugation, 

 pointing not only to a chemical modification but also 



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