130 THE PHYSIOLOGY OF REPRODUCTION 



or chromosomes of the cell nucleus conjugate together in pairs, 

 and that in all probabihty one member of each pair is a de- 

 scendant of a chromosome derived from the father, while the 

 other rhember is descended from a corresponding maternal 

 chromosome. 1 The possible significance of this conjugation of 

 chromosomes is referred to on a later page (p. 196). In the 

 subsequent maturation division the chromosomes again separate.^ 



The changes involved in the formation of the first polar body 

 are in most respects similar to those of ordinary cell division. 

 The oentrosome, which hes in the cytoplasm, divides, and the 

 two daughter centrosomes thus produced travel to opposite 

 sides of the nucleus. In the meantime, the latter forms a 

 spindle composed of the chromosomes, the nuclear membrane 

 having disappeared. Each centrosome becomes surrounded by 

 a system of rays, and in this way the attraction spheres are 

 formed. The chromosomes next arrange themselves equatorially 

 between the attraction spheres, each one having now split into 

 two parts. Half of these migrate towards each centrosome, 

 and the nucleus becomes divided. One of the daughter nuclei, 

 together with a thin investment of protoplasm, is extruded from 

 the ovum. This is the first polar body, which is therefore a 

 product of unequal cell division. Subsequently to extrusion 

 it sometimes divides into two. After the formation of the first 

 polar body, the ovum again divides in the same unequal fashion, 

 and the second polar body is formed and extruded. The polar 

 bodies undergo degeneration. Meanwhile the nucleus of the 

 ovum once more becomes surrounded by a membrane and enters 

 upon a resting stage. 



The process of formation of the second polar body differs 

 from that of the first in that the chromosomes do not undergo 

 spUtting. Consequently the nucleus of the mature ovum con- 



1 The observations of this author, together with those of Sutton, McClung, 

 Wilson, &o., point to the conclusion that all the nuclei in the somatic cells 

 contain two parallel series of chromosomes (paternal and maternal). 



' In the reduction process each pair of fused chromosomes becomes 

 divided into a group of four bodies united by linin threads. These are the 

 tetrads or " vierergruppen." It follows that the number of tetrads in any 

 particular species is always one-half the number of somatic chromosomes. 

 Thus, if the somatic cells contain sixteen chromosomes, the number of tetrads 

 formed is eight, while, as shown in the text, the number of chromosomes in 

 the mature germ cells (after reduction) is also eight. 



