THE FUNDAMENTALS OF MENDELISM 35 



their centromeres lying in a plane across the equator of the spindle (the 

 metaphase stage); the centromeres clearly repel one another and so do 

 the chromosomes as a whole, so that the centromeres lie as far apart as 

 possible consistent with being within the spindle, while the repulsion 

 between the chromosomes causes the larger ones to lie on the edges of 

 the equatorial plate, with the smaller ones more centrally placed. 



The centromeres then divide into two, and, the daughter centromeres 

 repelling one another, the chromatids are pulled apart and begin to 

 move towards the poles (the anaphase stage). The repulsion of the 

 centromeres seems insufficient to move the chromatids far apart and 

 the later stages of their movement is caused by the narrowing and 

 elongation of the material situated between the two groups of chromo- 

 somes, which forms the so-called "stem-body. "^ 



When the chromatids reach the two poles (the telophase stage) the 

 cell divides into tv^o^ and two new resting nuclei are reconstituted. The 

 individual chromatids of this division are now to be reckoned as chromo- 

 somes, and in the ensuing interphase each one will manufacture a 

 partner, or, if one likes to put it so, each one will become spUt longi- 

 tudinally, so as to appear in the next prophase as a double body. 



5^. Meiosis 



Meiosis consists of two successive cell divisions and takes place only 

 in the formation of the gametes in animals or spores in plants. The 

 daughter cells which are formed from it contain only one of each kind 

 of chromosome. They are said to contain the haploid number, which 

 is half the diploid number. This reduction in number, in consequence 

 of which the first division of meiosis is often called the reduction 

 division, is brought about by the pecuharities of the prophase, which 

 is more complicated than that of mitosis and is divided into several 

 sub-stages. 



{a) Leptotene. — The chromosomes appear as single unpaired threads, 

 in the diploid number. 



{b) Zygotene. — Similar chromosomes come together in pairs side by 

 side. 



(c) Pachytene. — Each chromosome splits longitudinally into two 

 half-chromosomes or chromatids. 



(d) Diplotene. — The four chromatids which were closely associated 

 at pachytene fall apart in pairs, the two pairs being held together by 



^ Belar 1927. 2 por a discussion of cell-division, see Gray 1931. 



