CAUSES OF XON-PAIRING 163 



Orgyia hybrid not only associations of three (as in Phragmatobia) 

 but also of larger numbers are found. Their details cannot be 

 made out, but interchange or translocation is to be inferred. In a 

 Dicranura hybrid definite chains can be made out (Federley, 1915). 



Ja ^ 



^y^ 





Fig. 56. — a, Crepis marschalH, 2n = 8. c, C. leontodontoides, 2n = 10. 

 h, the hybrid at mitosis, d, the hybrid at first metaphase ; two 

 unequal pairs and five unpaired chromosomes. X 1800. {a and 

 c from Babcock and Navashin, 1929 ; h and d from Avery, 

 1930.) Note that the chromosomes of C. leontodontoides are 

 larger in the hybrid owing to a change in genotypic control. 



Closer study of many hybrids amongst those that will be classified 

 as " undefined " would probably reveal analogous results and 

 make it possible to specify some of the differences that constitute 

 their hybridity. 



3. THE CONDITIONS OF METAPHASE PAIRING 



Before turning to more complicated kinds of hybrids we shall do 

 well to consider how far observations of simpler kinds will help us. 

 These show the following conditions of chromosome pairing. 



(a) Where the chromosomes of a diploid organism can be seen 

 at mitosis to consist of different pairs of identical chromosomes, the 

 chromosomes associated in pairs at meiosis are always seen to be 

 identical. Since the pairs segregate at random with regard to one 

 another their members must be identical chromosomes and pairing 



