DIFFERENTIATION OF STRUCTURE 23 



each group has nearly reached the centrosome, the whole 

 cell divides into two, each of which has a centrosome 

 and half the chromatin of its parent. The chromosomes 

 of the new cells either break up or unite, and in every 

 case form a fresh network or a new spireme. The 

 spindle fibres disappear and the karyokinetic cycle 

 begins again. 



This is the simplest form of karyokinesis. Many 

 complications occur in the formation of the reproductive 

 cells. In plants, again, there may be no centrosome 

 present. The whole question is full of uncertainties, 

 and the actual forces that bring about these compli- 

 cated changes are little understood. 



CHAPTER V 



DIFFERENTIATION OF STRUCTURE AND 

 DIVISION OF LABOUR 



A CURSORY glance at living organisms shows that we 

 can arrange plants and animals in two series, in each 

 of which there is a gradual transition from simple to 

 more complex forms. This does not mean that we are 

 able to connect each step in this series by an infinite 

 number of fine gradations, but that on the whole there 

 is a gradual increase in complexity as we advance from 

 what are called the lower to what we call the higher 

 forms of either plant or animal life. What the exact 

 benefits of this increase in complexity are, it is exceed- 

 ingly difficult to say, as we find extremely simple forms 

 living and thriving and holding their own side by side 

 with the more highly complex forms. 



