CHANGES IN THE OVARY 125 



hours after copulation, when the supply of nourishment has been 

 entirely cut off, the two polar bodies are formed, and the ovum 

 becomes mature.^ 



The essential facts about the maturation process were first 

 ascertained by van Beneden ^ in Ascaris, and were afterwards studied 

 more fully by Boveri.* Subsequently Montgomery* has elucidated 

 the process still further by showing that prior to the formation of 

 the first polar body the chromatin filaments or chromosomes of the 

 cell nucleus conjugate together in pairs, and that in all probability 

 one member of each pair is a descendant of a chromosome derived 

 from the father, while the other member is descended from a corres- 

 ponding maternal chromosome.^ The possible significance of this 

 conjugation of chromosomes is referred to on a later page (see 

 p. 201). 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 

 centrosonie, which lies 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, 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 



» Of. Thomson (A.), "The Maturation of the Human Ovum," and "The Eipe 

 Human Graafian Follicle," Jour, of Anat., vols. liii. and liv., 1919 and 1920. 



2 Van Beneden, " Recherches sur la Maturation de FCEuf," Arch, de Biol., 

 vol. iv., 1883. 



3 Boveri, " Zellenstudien," Jenaisclie Zeitsch., vol. xxi., 1887. 



* Montgomery, " Some Observations and Considerations upon the Matura- 

 tion Phenomena of the Germ-Cells," BM. Bull., vol. vi., 1904. 



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

 Wilson, etc., 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. 



