GAMETES AND FERTILIZATION 



9 



ucts of her digested food. It is brought in through the blood 

 vessels of the theca, absorbed by the follicular cells and trans- 

 ferred by them to the ovum. Within the ovum this material is 

 elaborated into deutoplasm. 



Maturation, Ovulation and Fertilization. When the full 

 allotment of deutoplasm has accumulated in the ovum the 

 nucleus undergoes its first maturation division. Maturation 

 is a process occurring before fertilization, in which there is 

 an equal mitotic division of the nucleus of the ovum but a 

 markedly unequal division of the cytoplasm and its contents. 

 The result of this division is the formation of one very large cell 

 containing the entire dower of deutoplasm and one very small 

 cell containing practically no deutoplasm. This small cell is call- 

 ed a polar body because it is budded off at the animal pole of the 

 ovum. Since this unequal division of the ovum typically 

 occurs twice we speak of the first and second 

 maturation divisions and of the first and second 

 polar bodies. 



In one of these maturation divisions the 

 chromosomes do not split at the metaphase stage 

 as happens in ordinary mitoses. Instead, half of 

 the original number of chromosomes migrate 

 bodily to each pole of the spindle, with the result 

 that each daughter nucleus receives but half the 

 number of chromosomes normal for the somatic 

 cells of the species. Such a modified mitotic 

 division is known as a reduction division. After 

 the maturation divisions, one of which is a reduc- 

 tion division, the nucleus of the ovum now ready 

 for fertilization, is called the female pronucleus. 



Although maturation in the male sex cells 

 differs in some respects from the maturation of 

 the ovum, there also, a reduction division occurs. 

 The result is that the nucleus of each matured 

 cell contains but half the species number of chromo- 

 somes. When in the process of fertilization the nucleus of the 

 male cell unites with the female pronucleus the full species 

 number of chromosomes is restored. 



At about the time of the first maturation division the follicle 

 ruptures, and the liberated ovum passes into the oviduct. If 



FIG. 2. 

 S permatozoon 

 of the pigeon. 

 (After Ballo- 

 witz.) 



