142 CYTOLOGY chap. 



chromosomes composing it, so that the V (AB) goes to one pole, and the 

 two non-Unked chromosomes (A and B) to the other (Fig. 68, D). 



In the case of the individuals with 33 chromosomes — i.e. with two V's, 

 or, in other words, in which linkage has occurred in both homologous 

 couples — we find instead of the open tetrapartite V in metaphase I. a 

 closed tetrapartite ring Hke the usual type of bivalent formed by syndesis 

 of two V's (cf. Lepidosiren, Fig. 16). 



Other similar cases could be cited, e.g., Notonecta (Browne, 1913). 

 Here w = 13 or 14, the former number being produced by the linkage of 

 two chromosomes which are separate in the latter. 



\ Fig. 69 illustrates five types of chromosome complexes found in 

 various species of the genus Drosophila, with their possible relationships 



Fig. 69. 



Five types of chromosome complex found in the genus Drosophila, showing their probable relationships. 



(After Metz, J.E.Z., 1914.) 



(Metz, 1914). The remaining four types are all derivable from type I. by 

 (i) breaking across of one or both pairs of V's, and (2) disappearance, or 

 attachment to another pair, of the pair of very small chromosomes. More 

 recently (1916) Metz has added several other types, all, however, simply 

 related to the above. 



So far we have dealt with linkage or fragmentation of chromosomes 

 permanent for the individual or species. A fragmentation of the type 

 chromosomes, leading to a variation of chromosome number in different 

 tissues or cells of a given individual, has also often been observed or 

 inferred. 



The classical case is that of A scar is megalocephala, in which the 

 somatic chromosomes undergo fragmentation, so that somatic mitoses 

 exhibit far smaller and more numerous chromosomes (about 60 in ^. m, 

 univalens) than do those in the germ track (see Chapter III.). It is note- 

 worthy that Payne (1913 h) found that subjecting eggs to radium emana- 

 tion causes the chromosomes in the germ track also to fragment. 



