THEORY OF EVOLUTION 91 



thousand-fold in every organ. We call this 

 unit a cell ( fig. 45 ) . 



The egg is a cell, and the spermatozoon is a 

 cell. The act of fertilization is the union of two 

 cells (fig. 47, upper figure). Simple as the 

 process of fertilization appears to us today, its 

 discovery swept aside a vast amount of mys- 

 tical speculation concerning the role of the 

 male and of the female in the act of procreation. 



Within the cell a new microcosm was re- 

 vealed. Every cell was found to contain a 

 spherical body called the nucleus (fig. 46a). 

 Within the nucleus is a network of fibres, a 

 sap fills the interstices of the network. The net- 

 work resolves itself into a definite number of 

 threads at each division of the cell (fig. 

 46 b-e). These threads we call chromosomes. 

 Each species of animals and plants possesses 

 a characteristic number of these threads which 

 have a definite size and sometimes a specific 

 shape and even characteristic granules at dif- 

 ferent levels. Beyond this point our strongest 

 microscopes fail to penetrate. Observation has 

 reached, for the time being, its limit. 



The story is taken up at this point by a new 



