82 PRINCIl'LES OF ANIMAL BIOLOGY 



DISCUSSION 



The physiological causes of cell division are still problematic. One 

 of the suggested causes of the initiation of the process is change in the 

 surface tension of the cell. Evidence in support of this view is believed 

 by some biologists to be found in the fact that unfertilized eggs of many 

 aquatic animals may be caused to segment (divide) if they are subjected 

 to certain chemical treatments. Such treatment reduces surface tension. 

 If it were regularly true that this change in surface tension were 

 localized, the phenomenon might offer an explanation of cell division. 

 Another theory connects the initiation of cell division with the well- 

 known capacity of colloidal substances, mentioned in the preceding 

 chapter, for changing from the liquid to the semi-solid state and vice 

 versa. A rhythmical change in the viscosity of the protoplasm of cer- 

 tain dividing eggs has been demonstrated, the protoplasm becoming 

 more solid during the prophases and more liquid during the later stages; 

 and when these changes in viscosity are artificially prevented, the divi- 

 sion of the eggs is arrested. Whether the change in viscosity is a 

 cause, either direct or indirect, of cell division, or is merely an effect 

 of some other factor which causes division, is uncertain. A change in 

 the rate of oxidation of substances in the protoplasm has likewise 

 been proposed as an agency causing cell division, and it is not im- 

 possible that electrical phenomena are also concerned with the process. 

 It is stated that cell division tends to preserve a certain relationship 

 between the bulk of the protoplasm of the cell and the area of its 

 surface, but this statement throws no light on the factors which initiate 

 cell division and thus bring about the readjustment between the bulk and 

 surface area of the cell. 



Universality of Cell Division. — That cells originate only from pre- 

 existing cells was affirmed in an earlier part of this chapter. Evidence 

 secured from the observations of an enormous number of investigators 

 working with both plants and animals substantiate that statement. No 

 trustworthy evidence has been secured to show that cells originate in 

 any other way than from cells or by any other method than cell division. 

 Biologists are thus forced to accept the idea of the continuity of proto- 

 plasm. Evidence for the continuity of certain specialized cell organs is 

 also at hand. Plastids, such as chloroplasts, originate only from existing 

 plastids by fission. Other cell organs besides chromosomes and plastids 

 are known to divide and thus perpetuate themselves, but their mainte- 

 nance and growth depend upon their relation to the cell. It is this 

 division of the cell and its organs, and especially the equal qualitative 

 division of the chromosomes, which furnishes the mechanism l)y which 

 the two cells resulting from the division of a parent cell resemble each 



