IMPREGNATION OF THE OVUM. 49 



mitotic figure was developing. In most cases a second polar globule is produced 

 a short time after the first. 



When this occurs, the nuclear remnants in the ovum do not reconstitute 

 themselves into a membranate nucleus, as occurs in ordinary cell division, but 

 they change directly into a second spindle, which lies, as did the first, within the 

 protoplasm of the animal pole. This second spindle occupies an oblique posi- 

 tion, or may even be parallel with the surface. But it also soon takes up a radial 

 position and produces a second polar globule in similar manner to the first. The 

 second globule is usually smaller than the first. 



It may also happen that the first polar globule may itself divide, so that 

 three polar globules appear. 



The Formation of the Female Pro-nucleus. — The nuclear material, which 

 remains in the main ovum after the separation of the polar globules, is known as 

 the female pro-nucleus. The nuclear remnant lies close to the animal pole and 

 in clear protoplasm. The details of its further history vary according to the 

 species of animal. Three tendencies are known to affect the pro-nucleus : viz., 

 to move toward the central position in the ovum, to unite with the male pro- 

 nucleus as soon as that is formed out of the spermatozoa which enters the ovum, 

 and to assume the character of a membranate nucleus. As the time of the for- 

 mation of the male pro-nucleus is variable, the other tendencies being more 

 constant, the exact history of the female pro-nucleus may be said to depend prin- 

 cipally on the appearance of the male pro-nucleus. The earlier that event, the 

 less does the female pro-nucleus move centrifug'ally and the less does it assume 

 the membranate form. Even among mammals there is variation. 



Time of Maturation. — The time when the polar globules are formed varies 

 according to the animal, and may be before or after the egg-cell leaves the ovary. 

 In placental mammals, maturation always begins, so far as known, in the ovary, 

 and is said in some cases to be completed there. But in other cases it is cer- 

 tainly completed only after ovulation or when the ovum has passed into the 

 Fallopian tubes. 



Impregnation of the Ovum. 



Impregnation is the union of the male and female elements to form a single 

 new cell, capable of initiating by its own division a rapid succession of generations 

 of descendent cells. The process of union is commonly called the entrance of the 

 spermatozoon into the ovum. The new cell is called the impregnated or ferti- 

 lized ovum. 



In all multicellular animals impregnation is effected by three successive 

 steps: (i) The bringing together of the male and female elements; (2) the 



