THE PROCESS OF FERTILIZATION 



299 



dealing with essentials and not with isolated and, so to speak, chance 

 details, but I must emphasize the fact that the same process of reduc- 

 tion of the number of chromosomes takes place in this or an analogous 

 manner in all animal ova, and can be demonstrated also in most of 

 the chief groups of plants. Whether it be, as many have maintained, 

 that the reduction is not always first effected by the 'maturation 

 divisions,' but in some cases takes place earlier in the primitive 



Red.H 



Fig. 76. Diagram of the maturation divisions of the ovum. A, primitive germ- 

 cell. B, mother-egg-cell, which has grovyn and has doubled the number of its 

 chromosomes. 0, first maturation division. D, immediately thereafter ; Bki, the first 

 directive cell or polar body. B, the second maturation spindle has been formed ; the 

 first polar body has divided into two (2 and 3) ; the four chromosomes remaining in 

 the ovum lie in the second directive spindle. F, immediately after the second 

 maturation division ; i, the mature ovum ; 2, 3, and 4, the three polar cells, each of 

 these four cells containing two chromosomes. 



egg-cell^, so much is certain, that the nuclei which come together 

 for 'fertilization' only contain half the normal number of chromo- 

 somes, and this is true not only of the ovum but also of the sperm- 

 nucleus. 



Arguing from general considerations, but especially from the 

 theory which regards the chromosomes as the bearers of the hereditary 

 substance, I had come to the conclusion, before there was any full 

 ' See the discussion of this point in chapter xxii. 



