called a reduction mitosis, as distinguished from all 

 preceding and all succeeding divisions, which are 

 called somatic mitosis. The significance of this re- 

 duction has led to many theories. During the 

 process the nucleus loses its membrane, is much re- 

 duced in size, and is now known as the female pro- 

 nucleus. All these preliminary changes are known 

 as maturation of the ovum. Without fertilization, 

 further development of the ovum does not seem possi- 

 ble in higher forms, and the cell is invariably lost. 



Fertilization. By this is meant the union of a 

 spermatozoon with the ovum, or, more technically, 

 the union of a male and a female pronucleus. This 



Fig. 4. Diagram of the division of the frog's egg (Hertwig): 

 A, stage of the first division. B, stage of the third division. The 

 four segments of the second stage of division are beginning to be divi- 

 ded by an equatorial furrow into eight segments; p, pigmented surface 

 of the egg at the animal pole; pr, the part of the egg which is richer in 

 protoplasm; d, the part which is richer in deutoplasm; sp, nuclear 

 spindle. 



union takes place in the upper part of the oviduct. 

 Maturation always precedes fertilization. But in 

 lower forms experiments upon unfertilized eggs in 

 the absence of spermatozoa have resulted in the 

 development of embryos, or larvae, and in a few in- 



