SIGNIFICANCE OF CONJUGATION 231 



eight, and at the first division two of these become the 

 meganuclei and two the micronuelei of the daughter forms. 

 In P. bursaria the original meganucleus does not break up 

 as it does in P. caudatum, and it is doubtful whether it is 

 eventually resorbed or whether it fuses with one of the new 

 meganuclei. 



The phenomena of conjugation in Paramecium caudatum 

 are worthy of most careful study, for they have been worked 

 out more carefully in this species than in any other Protozoon, 

 thanks chiefly to the brilliant researches of Maupas and Calkins. 

 Comparing the course of events with those observed in other 

 Protozoa, it will be observed that in Paramecium there is no 

 distinction of sex as far as the two gametes are concerned. 

 Both are exactly alike and equal in size. But there is an 

 indication of differentiation in the micronuclear products, so 

 that we have not hesitated to call the active product the male 

 and the passive one the female pronucleus. Again, it is remark- 

 able that the conjugation is only temporary, the two gametes 

 separating and dividing, each on its own account, after the 

 mutual transfer of micronuclear material has been effected. 

 There is no fusion of cytoplasm, and no zygote is formed. 



Perhaps the most striking result of Maupas' and Calkins' 

 researches is the evidence, more surely founded than in the 

 case of Polytoma or Copromonas, that in Paramecium and a 

 large number of other ciliate infusoria there is a definite limit 

 to the reproductive faculty, and when this limit is reached 

 death ensues if conjugation does not intervene to restore the 

 lost fertility. It would appear that in these lowly organised 

 creatures there is, as in the higher animals, a period of growth, 

 of bloom, and of decay ; that the organism wastes and wears 

 out in the exercise of its functions, and requires rejuvenation 

 from time to time in order that its activities may not be brought 

 altogether to a stop. This rejuvenation is effected, in a 

 manner unknown to us, by the act of conjugation, and Para- 

 mecium shows better than any other example, that the essential 

 thing in conjugation is the mingling of nuclear material derived 

 from two different individuals. 



In a Protozoon like Paramecium or Polytoma there is a 

 distinct life cycle, which comprises the alternation of a sexual 

 or conjugate generation with a series of asexual generations 

 reproducing their like by simple binary fission. The products 



