REPRODUCTION. 413 



menstrual period, suitable for the reception and development of the 

 ovum if fertilisation takes place. In many of the lower animals a 

 somewhat similar change in the uterus, known as the oestrus, occurs at 

 certain seasons of the year, and is 'accompanied by ovulation and sexual 

 activity. 



Maturation of the Ovum. After being discharged from the ovary 

 the ovum undergoes certain changes known as maturation. When a 

 cell divides in other (somatic) tissues the process is initiated by changes 

 in the nucleus and is known as mitosis. The network of chromatin in 

 the nucleiis breaks up into a number of segments called chromosomes, 

 each composed of rows of granules ; the number of chromosomes thus 

 formed is constant for every somatic cell for a given species of animal, 

 and in man the number is sixteen. The next stage consists in the 

 splitting of each chromosome longitudinally into two halves, which 

 travel to opposite ends of the cell. While this is taking place the cell 

 protoplasm constricts and then divides between the nuclei to form two 

 cells, each of which contains the same number of chromosomes as the 

 original cell. Finally, the chromosomes of each daughter cell fuse into 

 a single chrornatin filament. 



In the germ cells a different form of cell division takes place, and is 

 known as heterotype mitosis. The ovum divides twice. The daughter 

 cells from the first division differ greatly in size, and the smaller 

 one, called the first polar body, breaks up and disappears. The 

 characteristic feature of this division is that the number of chromosomes 

 formed is only half that occurring in a somatic cell in the same animal. 

 The larger cell, which contains only half the normal number of chromo- 

 somes, divides again to form two daughter cells, one of which is small, 

 the second polar body, whereas the other is large and constitutes the 

 mature ovum. 



A similar process takes place in the formation of spermatozoa, 

 except that the four daughter cells, spermatids, formed from the sper- 

 matocytes, all develop into spermatozoa ; and in man each spermatozoon 

 and mature ovum contains eight chromosomes, whereas the somatic 

 cells contain sixteen. 



Fertilisation takes place as a result of the introduction of sperma- 

 tozoa into the vagina during the act of coitus. The motile spermatozoa 

 travel into the uterus and Fallopian tubes, where they may live for 

 some days. If a spermatozoon penetrates into an ovum, it loses its 

 tail, changes take place in its head and neck, and it is converted into a 

 male pronucleus, which fuses with the nucleus of the ovum (female 

 pronucleus) to form a new cell containing the normal number of 

 chromosomes ; this process constitutes fertilisation. 



