2oo Education through Nature 



Like the microscopic organisms, the germ- cells 

 have the power to divide. This power ceases, how- 

 ever, after a certain number of divisions, when the 

 ovum, like the infusorian, unites with another cell, as 

 in conjugation. This process is called pollination in 

 plants and fertilization or fecundation in animals. 

 The effect of this is to cause the ovum to divide, 

 not into separate cells as previously, but into cells 

 that remain permanently associated after cell divis- 

 ion. Consequently a colony of what at first seems 

 like similar cells arises. This repeated division con- 

 tinues; but the resulting cells forming the colony 

 gradually become different from one another, like 

 cells becoming aggregated into groups, constituting the 

 tissues of all higher plants and animals. In higher 

 plants the final result of this cell multiplication is 

 the formation of such tissues as bark, cambium, wood, 

 bast, sieve, and tracheary tissues; in animals there 

 result skin, muscle, bone, glands, epithelium, etc. 

 There are two ways by which we can account for this 

 heterogeneity arising out of comparative homogeneity. 

 First, we may assume that the daughter cells arising 

 from cleavage (as this fc n of cell division is called) 

 are not really similar but eally different as a result of 

 unequal division of the mother cell; second, we may 

 assume that the cells are exactly similar, and that they 

 gradually become different because of the varying 

 conditions amid which they are placed. In that case 

 we must assume that the constantly increasing multi- 

 plicity of cells introduces a new factor into the life 

 of each cell, inasmuch as it now reacts, not only to 

 external influences, but all the cells, also, react on 

 each other. Every increase in the number of cells 

 causes a change in the environment of every cell, and 

 necessitates a new and different reaction on the part 

 of every cell. Each cell, we will say, is now a member 

 of a social body. Every reaction, we have seen, 



