208 BIOLOGY OF THE PROTOZOA 



example, were 279 and 263 generations old at the beginning of the 

 experiment, the single individual isolated daily in a drop of medium 

 divided 60 times in sixty days; with 4 individuals in a drop, each 

 divided only 31 times. Series 120 and 121 were 12 and 10 genera- 

 tions old, and each solitary individual divided 86 and 107 times in 

 the same sixty days, and with the same medium freshly made each 

 day. From this table it is apparent that the division rate is reduced 

 by the presence of more than one individual to a drop. Furthermore, 

 the reduction of the division rate under such conditions is much less 

 for "y° un g" individuals than for old. 



Substances making up the composition of living protoplasm are 

 constantly manufactured. Such substances, usually in the form of 

 granules, grow to a certain limit of size and each then divides. Evi- 

 dence for this is apparent only in the more obvious of the proto- 

 plasmic elements such as plastids, kinetic elements, chromomeres, 

 etc., the division of which has been mentioned in the preceding pages. 

 Finally the grand aggregate, the cell itself, divides as a last expres- 

 sion of the series of events that have taken place. It is evident 

 that such division of the cell as a whole constitutes only a small 

 part of the phenomena of reproduction and perhaps not the most 

 important part. While most of the elementary granules, apart 

 from those enumerated above, which make up the bulk of proto- 

 plasm, cannot be followed from their smallest stages to the stage 

 when they become visible, it is not inconsistent with the idea of 

 continuity from generation to generation to regard even the smallest 

 as retaining its integrity and reproducing itself by division. "For 

 my part I am disposed to accept the probability that many of 

 these particles, as if they were submicroscopical plastids, may have 

 a persistent identity, perpetuating themselves by growth and mul- 

 tiplication without loss of their specific individual type" (E. B. 

 Wilson, 1923). 



While the division of a single granule results in the formation of 

 two probably identical granules of the same substance, the division 

 of aggregates of granules of different substance may or may not 

 result in identical daughter aggregates. The nucleus is such an 

 aggregate which, by ordinary equation division, is probably divided 

 into two identical halves, but in meiotic divisions the products of 

 the nucleus are different, visible evidence of which is shown by the 

 history of the sex chromosomes and by the results in modern 

 genetics. It is entirely possible that differentiations may arise from 

 such inequalities in nuclear division (see Chapter IX). 



The cytoplasm of the cell, likewise, is such an aggregate, made up 

 of all the different substances variously distributed, which compose 

 living protoplasm. If all the granules were equally distributed at 

 division to the daughter cells, as are nuclei and many kinetic ele- 

 ments, then the products of cell division might be identical. Mor- 



