106 THE BIOLOGICAL PROBLEM OF TO-DA Y 



being dependent upon the development of the 

 whole. 



The power of the egg to multiply by division is a 

 chief and most important factor in the production 

 of complexity during the course of development. 

 It is only because the nuclear material, by a series 

 of intricate, chemical changes, assimilates reserve 

 material from the egg and oxygen from the atmo- 

 sphere that it can give rise to continually increas- 

 ing complexity within itself. The increase in bulk 

 results in a cleavage into two, four, eight, and 

 sixteen pieces, and so forth. The cleavage produces 

 a constantly changing distribution in space of the 

 nuclear material. The two, four, eight, and sixteen 

 nuclei that arise by division diverge from each 

 other and take up new positions inside the egg, in 

 definite relations to each other. At first the 

 particles of the egg were arranged around the 

 fertilised nucleus, which was a single centre offeree ; 

 they become grouped around as many centres of 

 forces as there are nuclei, and so become segregated 

 into as many cells. Clearly enough, the egg, in its 

 single-celled condition, changes its quality in a 

 marked degree when it becomes multicellular, even 

 although the change has occurred by doubling 

 division. 



This, so clear in the early stages of development, 

 continues to occur throughout the later stages of 

 growth. The continued cell-multiplication causes 

 not only changes of bulk, but also from time to 

 time changes in quality ; for each shape is bound 

 up with definite conditions. When the conditions 



