848 THE PHYSIOLOGY OF REPRODUCTION. 



tube its nucleus undergoes the changes preparatory to a mitotic 

 division. These changes are essentially similar to those of ordi- 

 nary cell division as represented schematically in Fig. 269. The 

 nucleus at first presents the ordinary chromatin network, and in 

 the cytoplasm lies the minute structure known as the centrosome. 

 This latter divides into two daughter-centrosomes (6) which move 

 to opposite sides of the nucleus and become surrounded by rays, 

 each centrosome with its radiating system forming an astro- 

 sphere. The chromatin material in the nucleus meanwhile has 

 collected into larger threads known as chromosomes (c), and 

 the nuclear membrane disappears (d). The number of chromo- 

 somes is definite for each species of animal. The chromosomes 

 arrange themselves equatorially between the astrospheres and then 

 each divides longitudinally into two parts (/) . These parts migrate 

 or are drawn toward their respective centrosomes (g, h, i), and this 

 division is followed by a separation of the cytoplasm into two parts. 

 Thus, two daughter-cells are formed, each containing the same 

 number of chromosomes as the parent-cell, but only half the amount 

 of chromatin material. The cell division results in a quantitative 

 reduction of the chromatin material. In ordinary cell division the 

 chromosomes again form a resting reticulum and a nuclear mem- 

 brane and the chromatin substance increases in quantity. In the 

 ovum a similar cell division takes place except that the daughter- 

 cells are very unequal in size ; one is very minute and is known as 

 the first polar body, the other as the ovum. After the formation and 

 extrusion of the first polar body, the ovum again undergoes division 

 into two unequal halves, giving rise to a second polar body. In 

 this division, however, the chromosomes are divided transversely, 

 and the ovum after the division is left with only half the number of 

 chromosomes characteristic of the species. In the formation and 

 extrusion of the two polar bodies the matured ovum has suffered 

 both a quantitative and qualitative reduction in chromatin ma- 

 terial, and is left with only half its number of chromosomes. Since 

 the first polar body after its separation may again divide into two 

 cells, the process of maturation results in the formation of four cells, 

 three of which are polar bodies and may be regarded as abortive 

 ova. The fourth, the matured ovum, retains practically all of the 

 original cytoplasm, but has lost a part of its chromatin material 

 and, according to Boveri, also its centrosome. The production 

 of these four cells may be represented, therefore, by a schema of 

 the kind shown in Fig. 270. The details of this process of forma- 

 tion of the polar bodies and of reduction in chromatin material 

 differ somewhat in different animals.* The process has not been 



* For details see Wilson, " The Cell in Development and Inheritance." 

 Second edition, 1900, New York. 



