172 CONJUGATION, MATURATION, AND FERTILIZATION 



successful. The conditions, as he outlined them, are, briefly: (1) 

 diverse ancestry of the conjugating cells; (2) scarcity of food; and 

 (3) sexual maturity. 



That diversity of ancestry has no great biological significance is 

 borne out by the facts of parthenogenesis, autogamy, and endogamy, 

 and on this ground alone might well be dismissed as a necessary con- 

 dition of fruitful conjugation. Not only in these instances, out in 

 exogamic fertilization as well, diverse ancestry is not essential. Thus, 

 in Paramedum aurelia (caudatum), which was one of the examples 

 cited by Maupas as an obligatory exogamous type, Calkins ('02) 

 showed that two cells removed by not more than eight or nine divisions 

 from the same ancestral cell, conjugated, and one of the exconjugants 

 gave rise to descendants through 379 generations of divisions. In 

 these experiments it was shown, furthermore, that fully as many con- 

 jugations between related forms were fruitful as between forms of 

 diverse ancestry. 



The second of Maupas' conditions, scarcity of food, seems to have 

 some connection with the ability to conjugate, although in no case has 

 it been proved that such a condition is a necessary factor. Certainly 

 in cultures of paramecium, or of any other ciliate, dividing forms 

 indicate the presence of food, and in such cultures conjugating and 

 dividing individuals may be found side by side, and Maupas himself 

 states that conjugating forms may still actively take in food. It is not 

 improbable that surplus of food, followed by starvation, may assist 

 in bringing about the protoplasmic conditions where conjugation is 

 possible. Changes in the density of the surrounding medium, and 

 changes in temperature, certainly act to this end, but all of such 

 conditions seem to be dependent upon a third condition, sexual 

 maturity. 



/ 



Maupas' third condition of conjugation, sexual maturity, seems to 

 be quite probable, provided we mean by sexual maturity the appro- 

 priate chemical and physical condition of the protoplasm when con- 

 jugation is possible. The time element, which seems to be implied, 

 is not a necessary factor, however, for the proper conditions may 

 be induced by temperature and density changes in the surrounding 

 medium. 



Finally, it appears to be not improbable that the interpretation of 

 fertilization rests in the obscure chemical relations and hypothetical 

 enzymatic action of idiochromatin elements whose potency depends 

 more or less upon the diversity of environment of the conjugating 

 forms. Culture experiments upon some of the larger forms of protozoa, 

 while not proving such a theory, nevertheless seem to point in this 

 direction. Thus, Cull ('07) found that out of a total of 186 para- 

 mecium individuals from pond water, 70 per cent, continued to live 

 after conjugation, ?. e., were fruitful. Calkins ('02), on the other 



