EMBRYOLOGY OF THE CHICK 319 



Oogenesis. The ovum during its formation passes through three 

 stages. 



The first stage, that of division, takes place before the chick is 

 hatched, and, according to Bradley, comes to an end about the time 

 of hatching. This stage consists of the rapid formation of ova in 

 the female chick. In the second stage, which begins about the time 

 of hatching, there is an increase in the size of the units of the ovary, 

 accompanied by yolk formation. At this time each ovum is in its 

 own follicle (Fig. 81), and is surrounded by a layer of cuboidal 

 cells and a theca. The theca is formed from the adjacent fibrous 



FIG. 81. Section of ovum in a hen. i. Nucleolus. 2. Nucleus. 3. Liquor 

 folliculi. 4. Stratum granulosum. 5. Follicular cells. 6. Theca folliculi. 

 7. Peripheral stroma. 



stroma. The third stage, that of maturation, commences during the 

 development of the yolk and is complete after it has escaped to the 

 oviduct. Maturation consists of each cell's dividing into two un- 

 equal parts. In each division the cell is split into a small cell known 

 as the polar body, which is cast off and disappears, and a larger cell, 

 which is the ovum proper. In this process half of the original 

 chromosomes are cast off. 



Fertilization. The sperm travels rapidly; experiments have shown 

 eggs to be fertile laid twenty-four hours after service by a male. 



