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 



o 



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 



