176 FOOT-NOTES TO EVOLUTION. 



chromosomes from the egg nucleus and half from the 

 sperm, thus containing equal amounts from each par- 

 ent. The centrosome, which, as we have seen, is to be 

 regarded as the dynamic centre of the cell division, 

 comes from the spermatozoon alone; the egg, on the 

 other hand, furnishes the yolk and practically all of the 

 cytoplasm. 



After this preliminary outline of the facts of fertiliza- 

 tion we are in a better position to understand the details 



of a process which occurs in the develop- 

 The reduction ^^^^ ^^ ^^^^ ^^^ ^^ ^^jj^^ ^^^^_ 



of the chromo- , , , . ^ , , 



ly, the reduction of the chromosomes, 

 somes. •" 



The necessity for such a reduction is 

 evident from a moment's reflection. We have seen that 

 the number of chromosomes in the nucleus is a con- 

 stant and typical one for each animal and plant species 

 so far as known. As fertilization consists in the union 

 of two cells into one, from which the young organism 

 develops, it is plain that, were there no reduction, 

 the number of chromosomes would be doubled in each 

 succeeding generation. However simple this necessity 

 for reduction may appear, the minutiae of the processes 

 through which it is brought about, and the theoretical 

 significance of these facts, form the most involved prob- 

 lem of biology to-day. In a few forms, especially among 

 the lower Crustacea, the facts of the reduction are clear 

 and relatively simple ; in other forms they thus far stand 

 in direct contradiction, and, for the present, a compre- 

 hensive explanation applicable to all forms must be left 

 to further investigation. 



The significance of reduction turns upon the concep- 

 tion of a definite organization and individuality in the 

 chromosomes and the assumption that they represent 

 the physical basis of heredity — i. e., that they influence 

 and determine into what the fertilized egg shall develop. 



