Ch. VI, 4] NATURE OF FERTILIZATION 



285 



There comes, however, a break in the regularity of this 

 chromosome division. It occurs when the adult plant is 

 forming its own sex cells (pollen and embryo sac). At this 

 time one cell division, called the reduction division, yields 

 to the new cells only half the number of chromosomes which 

 had prevailed through the body. 

 The functional significance of the 

 reduction is perfectly clear, for if 

 the uniting sex cells contained the 

 full number of chromosomes, ob- 

 viously the number would double 

 in every generation, to their ulti- 

 mate enormous multiplication, 

 whereas by the reduction division 

 the number is kept constant. The 

 details of the reduction division are 

 complicated and not wholly under- 

 stood, but it occurs in such a way 

 as to give each sex cell one complete 

 set of chromosomes instead of the 

 double set which all body cells pos- 

 sess. These chromosomes, however, 

 (and this is a point of great con- 

 sequence in heredity), do not repre- 

 sent individual chromosomes which 

 occurred in the body cells, but are 

 reconstructed from them in such a 

 way as to include some material 

 from the father set and some from 

 the mother set, in combinations which apparently are due 

 only to chance, and never the same in any two. This mat- 

 ter is illustrated by the diagram of Figure 219, and is 

 apparently identical in every respect in plants and animals. 



Thus the principal consequence of fertilization seems to 

 consist in the introduction of complete sets of paternal and 

 maternal heredity-carrying chromosomes into every cell 



Fig. 197. — Flowers of 

 the Hazel (Corylus Avellana) ; 

 X \. The staminate flowers 

 hang in two large catkins, 

 and the pistillate flower 

 stands near the top of the 

 stem. (From Balfour.) 



