REPRODUCTION 67 



snow was still on the ground, and when I 

 again ventured out to cross the garden wall 

 only a hundred steps from me, I was as- 

 tounded to find that the fields of rye were in 

 the ear." This is the application and energy 

 that brought to light the mammalian egg. 

 The fact is a commonplace now, but let us 

 not forget that it took a genius to point it out 

 to us. 



The human egg is a minute spherical body 

 just about visible to the unaided human eye. 

 It has a diameter of about one fifth of a milli- 

 meter. Aside from a slight envelope, it is com- 

 posed of a mass of protoplasm containing some 

 fine yolk granules and a spherical nucleus 

 whose diameter is about one seventh or one 

 eighth that of the whole structure. Thus the 

 human egg, like that of many other animals, 

 exhibits all the characteristics of a simple cell. 



The two ovaries of a woman shortly after 

 the onset of puberty are estimated to contain 

 seventy-two thousand so-called primitive eggs, 

 of which, it is believed, not more than about 

 four hundred become mature and are dis- 

 charged. These mature egg cells are liberated 

 with more or less regularity over the period 

 of sexual maturity. This is ordinarily from 



