CH. LIX.] THE POLAR BODIES 829 



unequal parts. The first division produces an oocyte of the second 

 order and the first polar body, and the second, which takes place 

 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. In some casts, only one polar body is formed, that 

 is, only one division occurs. 



The unequal division is naturally associated with an eccentric 

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

 projects in the surface with a little surrounding protoplasm, and it 

 is the small process which becomes the polar body. 



One of the essential features of maturation is the reduction of 

 the number of chromosomes in the nucleus. It is well known that 



2 a 



2ci2 



Fio. 621. Diagram showing the stages in the maturation of the ovum when the first polar body 

 divides. A similar diagram would represent the formation of spermatids from a spermatocyst of 

 the first order. 1, Oocyte of the first order ; 2, oocyte of the second order ; 2a, first polar body ; 3, 

 mature ovum ; 3a, second polar body ; 2al, and 2a2, daughter cells of the first polar body. All the 

 last generation in the male would be spermatids of equal value. 



the nuclei of all animal cells, including germ cells and oogonia, con- 

 tain a definite number of chromatic particles. When maturation 

 commences in the oocytes of the first order, 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 Y-shaped 

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

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

 the end of the process the oocyte of the second order and the first 

 polar body both contain four chromosomes. This form of mitosis is 

 known as heterotype, whilst the ordinary form is called homotype. 

 The second division which produces the mature ovum and the second 

 polar body is of the homotype form, and the final result is that each 

 of the segments into which the oocyte of the first order has divided 



