REPRODUCTION 33 



becoming detached, and the cell substance passes into 

 solution to form the albuminous and carbohydrate con- 

 stituents of the milk. The fat droplets which collect in 

 the disintegrating part of the cell give rise to the milk 

 fat. The basal portions of the cell remain in position 

 without being detached, and subsequently develop fresh 

 processes, which in their turn become disintegrated. It 

 is believed, however, that some cells simply discharge 

 their fat droplets and other contents into the lumina, 

 while otherwise remaining intact." 



31. Fertilization of the ovum. We have seen how 

 growth may continue for a long period by successive 

 cell division. Indeed, in many of the simpler forms, 

 Wilson has pointed out that " as far as we can see from 

 an a priori point of view, there is no reason why, barring 

 accident, cell division should not follow cell division in 

 endless succession in the stream of life." 1 



In some of the very simplest forms, no sexual union 

 has so far been discovered. But, under normal conditions, 

 reproduction without sexual union is rare and practically 

 unknown in the higher animals. The impetus to growth 

 and cell division is not permanent and must be constantly 

 renewed. The stimulus to renewed growth is accom- 

 plished by the definite mixture of living protoplasm from 

 two entirely distinct individuals. This process of fertili- 

 zation involves a union of the ovum of the female and 

 the spermatozoon of the male. This union is the begin- 

 ning of the life of each individual and results not only 

 in energizing the protoplasm of the germ-cell, causing it 

 to divide and grow, but the admixture of germ material 

 from two different individuals introduces into the new 

 organism two distinct lines of inheritance. . 



i Wilson, "The Cell." 



