REPRODUCTION 207 



Each substance entering into the composition of living protoplasm 

 must manufacture new substance of. its own kind. All such sub- 

 stances, usually in the form of granules, grow to a certain limit of 

 size and each then divides. Evidence for this is apparent only in 

 the more obvious of the protoplasmic 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 expression 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 ele- 

 mentary granules, apart from those enumerated above, which make 

 up the bulk of protoplasm, 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 the these particles, as if they were submicroscopical 

 plastids, may ha^•e a persistent identity, perpetuating themselves 

 by growth and multiplication without loss of their specific individual 

 type" (E- B. Wilson, 1923). 



Wliile 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 equations division, is probably divided 

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

 the nucleus are different, visible e\-idence 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 XII). 



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

 of all the difl'erent substances variously distributed, which compose 

 living protoplasm. If all the granules were equally distributed at 

 di^•ision to the daughter cells, as are nuclei and many kinetic ele- 

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

 phological evidence that all granules are not thus equally distributed 

 is furnished by all budding and spore-forming types, and by forms 

 like Dileptus anser or Holosticha muUinuclcata, where the large 

 chromatin granules, while still in the process of division, are carried 

 bodily to one or the other daughter cell (Fig. 58, p. 11(3). 



Ileproduction whereby a type of organism is perpetuated and 

 distributed, is thus preeminently a process of division. In the last 

 analysis cell division is the only kind of reproduction known. 

 Potential individuals are contained in every germ cell, but germ 

 cells, like other cells, are formed by division and it follows that every 



