876 REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



without any resting-stage, results in the formation of the mature 

 ovum and the second polar body. Thus, when the two divisions are 

 completed, the mature ovum and two polar bodies lie inside the 

 zona pellucida. 



The unequal division is associated with an eccentric position 

 of the spindle. At each division one end of the spindle projects 

 on the surface with a little surrounding protoplasm, and it is this 

 small nodule which becomes the polar body (see fig. 561). 



One of the essential features of this maturation is the halving of 

 the number of chromosomes in the nucleus. When maturation com- 

 mences in the primary oocytes, an achromatic spindle is formed in 

 the usual way ; but instead of the ordinary number of chromosomes 

 appearing at its equator, only half that number are seen: for 

 example, if eight be the normal number of chromosomes, only four 

 appear. Further, each chromosome is not a slender V-shaped loop, 

 but a short, thick rod, or ring, or group of four particles. Neither 



2a 



2 02 



Pio. 562. Diagram showing the stages in the maturation of the ovum when the first polar body 

 divides. 1, Primary oocyte; 2, secondary oocyte; 2a, first polar body; 3, mature ovum; 3a, 

 second polar body ; 2al, and 2a2, daughter cells of the first polar body. 



does it split longitudinally in the usual way, but transversely ; and at 

 the end of the process the secondary oocyte and the first polar body 

 both contain four chromosomes. This form of mitosis is known as 

 heterotype, in contradistinction to the ordinary form which is called 

 Jiomotype. The second division which produces the mature ovum 

 and the second polar body is homotype, and the final result is that 

 each of the segments into which the primary oocyte divides contains 

 half the number of chromosomes present in the parent germinal 

 cell. In some cases, the first polar body divides at the same time 

 that the second polar body is formed; the whole process is repre- 

 sented in the schema in fig. 562. The nucleus of the mature ovum 

 is known as the female pronucleus. 



It should be added that the mature ovum is destitute of a 

 entrosome ; that is lost with the formation of the polar bodies. 



