96 Biology in America 



protoplasm weighing one kilogram. At the end of 30 days, 

 at the same rate, the number of kilograms would be 1 fol- 

 lowed by 44 zeros, or a mass of protoplasm a million times 

 larger than the volume of the sun." x 



But in many of these forms another process of reproduction 

 has developed, wherein two cells, or in any event two nuclei 

 play a part, a process known as conjugation or fertilization. 

 A simple example will illustrate this. If a single individual 

 of the ciliate Paramoecium be placed in a suitable culture fluid 

 it will divide rapidly, forming in a few days a countless 

 progeny. After a time if one examine a drop of the culture 

 he will likely find some of the individuals united in pairs. 

 They remain thus united for possibly 24 hours, after which 

 they separate and each resumes its rapid multiplication. 

 Briefly told, what happens during their union is as follows: 

 Paramoecium contains two nuclei, a larger, or macronueleus 

 and a smaller, or micronucleus. 2 The latter divides three 

 times and several of the parts thus formed disappear, but 

 two remaining. Of these one, the larger, remains quiescent, 

 while the other migrates across the protoplasmic bridge unit- 

 ing the two animals and fuses with the quiescent nucleus in 

 the other cell. This fusion nucleus then divides several times, 

 some of the daughter nuclei enlarging to form new macro- 

 nuclei which are distributed to the daughter cells, the old 

 macronucleus having disappeared in the meantime. Some 

 of the daughter nuclei degenerate, while one remains to form 

 a new micronucleus. We have here probably the beginnings 

 of sex as indicated in the difference of size and activity of 

 the two micronuclei, which fuse with each other during the 

 cell union. Externally however sexual difference between 

 the cells is not evident. 



The meaning of this process is not clear. It has been sup- 

 posed to have a rejuvenating influence upon the cells taking 

 part in it, but this interpretation is rendered doubtful by 

 recent experiments of Professor Jennings at Johns Hopkins, 

 who suggests that it is rather a means of producing variation 

 and thus leading to evolution. 



The union of similar gametes or reproductive cells is com- 

 mon among the algae. Frequently these cells bear cilia and 

 are motile, while the ordinary form is non-motile. In some 

 cases a slight difference in size between the conjugating 

 gametes is suggestive of the differentiation between egg and 

 sperm cell of higher forms. 



1 These figures are given by Morgan in " Heredity and Sex." He 

 disclaims responsibility however for the mathematical computation in- 

 volved. 



* There may be one or two of these latter. 



