38 THE EARLY DEVELOPMENT OF MAMMALS. 



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 spermatozoon which enters the ovum, and to assume 

 the character of a membranate nucleus. As the time of the formation 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 principally on the appearance 

 of the male pro-nucleus. The earlier that event, the less does the female pro- 

 nucleus move centrifugally 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 certainly 

 completed only after ovulation or when the ovum has passed into the Fallopian tube. 



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 fertilized 

 ovum. The process of fertilization in the mouse is described and illustrated in 

 Chapter V. 



In all multicellular animals impregnation is effected by three successive steps: 

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

 spermatozoon into the ovum and the formation of the male pro-nucleus; (3) fusion 

 of the pro-nuclei to form the segmentation nucleus. 



Meeting of the Sexual Elements. In all amniota the seminal fluid is transferred 

 from the male to the female passages during coitus, and spermatozoa are thereafter, 

 in mammals, found in the uterus. In default of actual knowledge it is commonly 

 believed that the spermatozoa make their way by their own motions into the Fallo- 

 pian tubes. The ovum, meanwhile (probably, in mammals, while completing its 

 maturation), travels down the tube. The meeting-point, or site of impregnation, 

 in placental mammals is about one-third way down from the fimbria to the uterus. 

 The exact spot is remarkably constant for each species. Nothing is known by 

 direct observation as to the site of impregnation in man, but there is no jeason to 

 suppose, as has unfortunately been often done, that the site is either variable or 

 essentially different from that in other mammals. 



The Entrance of the Spermatozoon into the Ovum. It is probable, in mammals 

 at least, that only one spermatozoon normally enters the yolk of the ovum and ac- 

 complishes its fertilization. It has been observed in those animals in which, as in 



