i 



PARAMECIUM 77 



These groups of chromatic material then become recognizable 

 as distinct nuclei (Fig. 31, e). The smaller nucleus might be 

 considered comparable to the male nucleus, the other the female. 

 The male nucleus migrates across the protoplasmic bridge between 

 the two animals (Fig. 31,7) and unites with the female nucleus of 

 the other conjugant (Fig. 31, g; Fig. 32, B), forming a fusion 

 nucleus (h). Thus is fertilization effected. 



The conjugants separate soon after fertilization (Fig. 31, g). 

 The macronucleus, which up to this time has remained at rest, 

 now assumes a vermiform shape, breaks up into small segments, 

 and then dissolves. The fusion nucleus of each conjugant, 

 shortly after separation, divides by mitosis into two (i), these two 

 into four (j) and these four into eight nuclei equal in size (k). 

 Four of these forming a group near the anterior end increase in 

 size and develop into macronuclei (/) ; the other four are grouped 

 near the posterior end. Three of these degenerate, the fourth 

 remains a micronudeus (/). The animal at this time then con- 

 tains four macronuclei and one micronucleus (m) . This micronu- 

 cleus divides into two (n). The whole animal then divides by 

 binary fission, each daughter cell securing two of the macronuclei 

 and one micronucleus (0). The micronuclei of the two daughter 

 cells divide again (p) , and another binary division results in four 

 cells each with one macronucleus and one micronucleus (q). An 

 indefinite number of generations are produced by the transverse 

 division of the four daughter cells resulting from each conjugant 



(71, 57, 54). 



Life Cycle (53). Enough is known of the life history of 

 Paramecium to enable us to give a brief sketch of its life cycle. 

 We must first define what we mean by an individual. It has been 

 pointed out that the entire series of cells which are produced by 

 binary division from the time of one conjugation to that of the 

 next ought to be compared with a many-celled animal, a Meta- 

 zoon (54). We have in another place (p. 14) shown that animals 

 pass through three phases of physical activity, youth, maturity, 

 and old age. To the period of youth we attribute a high degree 



