82 EVOLUTION AND GENETICS 



are produced (by these two divisions) from each cell 

 of the testis {jig. 34, i) . 



In the female the two divisions of the egg-cell are 

 very unequal {jig. 35), although the chromosomes 

 are distributed equally to all the cells. Thus at the 

 first division one cell is very small {jig. 35, c) and is 

 called the polar body. At the next division the polar 

 body divides again, and at the same time the egg 

 divides again also, producing another polar body 

 {jig. 35, d, e, /) . The three polar bodies and the egg- 

 cell are equivalent to the four spermatozoa, but only 

 the egg-cell undergoes further development — the 

 polar bodies disappear. Although only one cell sur- 

 vives, nevertheless there will be as many kinds of 

 mature eggs as there are kinds of sperm cells (with 

 respect to the distribution of the chromosomes), if, 

 as we now know to be the case, the distribution of 

 the chromosomes in the two final divisions (matura- 

 tion divisions) are the same in the eggs and in the 

 sperm-cells. When the eggs are fertilized, each by 

 one spermatozoon, the whole mimber of chromo- 

 somes is restored. 



The 3Iechanis?}i of McndeVs Two Laws 

 The behavior of the chromosomes at the time of 

 maturation of the egg- and sperm-cells furnishes a 

 mechanism for ^lendelian hereditv if the chromo- 

 somes are the bearers of the hereditary elements, and 

 if they maintain their integrity both during the rest- 



