vi] THE MEIOTIC PAIRING 87 



Nyssia zonaria\ In B. hirtarius the reduced germ-cells con- 

 tain 14 mostly rather large chromosomes of which two are 

 usually united, giving the appearance of 1 3 ; those of N. 

 zonaria have 56 small ones. The hybrid has 70, some large 

 and the rest small, in its spermatogonia. After the growth- 

 phase about 60 to 65 chromosomes emerge from the spireme, 

 a few of which are evidently double (bivalent), while the 

 majority are single (univalent), a few of these being large 

 and the rest small. At the first division the bivalents 

 divide into their components, and the rest divide equation- 

 ally, and probably all divide equationally at the second 

 division, producing spermatids with only a few less than 

 the somatic number of chromosomes. The explanation is 

 clearly that most of the chromosomes derived from the 

 one parent are too unlike any of those derived from the 

 other for pairing to be possible, so that hardly any pairing 

 takes place, and the chromosomes in the maturation 

 divisions are nearly as numerous as in the spermatogonial, 

 instead of only half as many. A few of the chromosomes 

 derived from the two parents, however, correspond suffici- 

 ently to be able to pair, and these give rise to the bivalents, 

 which seem to vary in number from five to about ten. 



Further evidence pointing in the same direction will be 

 given in the chapter on the cytological phenomena of sex. 



That the pairing and separation of the chromosomes in 

 the meiotic phase brings about the required result is per- 

 fectly evident, but it is not so clear why the reducing 

 division should always be followed by a second division of 

 the equational type, that is to say, in which the individual 

 chromosomes are split into longitudinal halves. A number 

 of hypothetical explanations of this second division have 

 been offered, but as most of them are quite devoid of 



1 HARRISON and DONCASTER, 1914. 



