v HOMOLOGOUS CHROMOSOMES 125 



called homologous chromosomes, pair together to form the bivalents. 

 The nucleus of the primary oocyte or spermatocyte can therefore be 

 written : 



AA+BB+CC + . . . 



At the reduction division the homologous chromosomes are separated, 

 as described in Chapter II., so that each gamete nucleus has the formula 



A+B+C+ . . . 



The diploid nucleus of the first formula is of course reconstituted at 

 syngamy. 



Thus it follows that one member of each pair of homologous chromo- 



A 



B 



FIG. 55. 



Illustrating the tendency of homologous chromosomes to lie near each other in somatic nuclei. (Muller, 

 A.Z., 1912.) A-C, polar views of equatorial plates; D, prophase. A, Eucomis bicolor; B, Albuca fastigiata 

 C, Galtonia candicans D, Dahlia coronata. Some of the pairs are numbered. 



somes in a diploid nucleus has been derived from the male and one from 

 the female parent. 



This morphological fact, together with its theoretical consequences 

 for heredity (to be discussed in the next chapter) was first pointed out 

 clearly by Sutton in the case of the insect Brachystola magna. 



The degree to which the chromosomes of a single nucleus differ from 

 one another in length varies greatly, and indeed in some species no 

 certain differences are detectable. Such species are of course of negative 

 value as evidence in this respect, the generalization being founded on 

 those numerous other forms in which the chromosomes exhibit marked 

 size differences. The seriation of the chromosomes according to size is 

 often facilitated by a tendency on the part of homologous chromosomes 

 to lie near or next to one another on the equatorial plate. This tendency 

 also varies in different species, in some indeed apparently not existing, 

 while in others (e.g., Yucca ; Muller, 1912) it is pronounced (Fig. 55). 



