REPRODUCTION AND GROWTH 



183 



The first segmentation division : 



The meaning of fertilization. Probably the most significant 

 aspect of fertiUzation is the stimulus of the entrance of the sperm 

 into the ovum. This " activates " the latter so that reproduction 

 of the ovum-cell, by mitotic division, is set up. (Nevertheless, 

 such activation is not absolutely necessary : see Sections 63— 68<3.) 

 But equally important, perhaps, may be the addition to the 

 ovum-nucleus of agencies carried by the sperm-nucleus : these 

 agencies add male potencies to the female egg-cell. 



66c. The Distribution and Determination of Sex. In 

 most multicellular animals that reproduce sexually the number 

 of male individuals is approximately equal to that of the females. 

 When this is apparently not the case we may suspect that the 

 life-histories of the males and females are so different that it 

 may be difficult to " sample " the population in question and find 

 the true ratio of males and females. 



Dwarf and complementary males. Nevertheless, there are cases 

 v^here the males are small, parasitic on the females, or otherwise 

 so modified that the true ratios of the sexes may not easily be 

 observed, or even may be greatly upset on development. 



In general, in wild nature, the female is the larger and more 

 active and longest-lived of the two sexes. The natural rates of 

 growth, habits and longevity may differ. This again may upset 

 the sex ratio. 



The determination of sex. The approximate equality of the two 

 sexes, and the fluctuations of the ratio, suggest that random causes 

 are involved in the establishment of maleness and femaleness in 

 an embryo. In multicellular organisms the condition of herma- 



