THE BEHAVIOUR OF INDIVIDUAL CHROMOSOMES 



87 



somatic elimination and thus give rise to mosaic patches.^ Some mosaics 

 in insects have arisen in a way other than by chromosome ehmination. 

 Doncaster showed that in Lepidoptera there are gynandromorphs 

 which are formed following fertiUzation of binucleate eggs (i.e. eggs in 

 which the second polar body is retained). Ilnd and Ilird chromosome 

 mosaics in Drosophila are probably not formed by elimination, though 

 IVth chromosome mosaics are. Stem^ has described a case of a mosaic 

 found in the Fi of a cross bet^\'een Stubble (III) and Haplo-IV Curly 

 (II); the mosaic was haplo-IV 4- ^^Sb in the head and left part of the 



Fig. 38. Plant Chimaeras. — A leaf of Solanum sisymbrifolium, 6 of S. nigrum> 

 C a periclinal chimaera with an interior of S. nigrum above which is one layer of 

 tissue from S. sisymbrifolium. D a partial periclinal ("mericlinal") in which the 

 superficial layer of S. sisymbrifolium tissue is present only in the shaded region. 

 Note the interaction of the tissues in determining the shape of the leaf. 



(From Jorgensen and Crane.) 



thorax and haplo-IV + ^^Cy in the right part of the thorax and abdo- 

 men. The first constitution must have originated by the fertiUzation of 

 an Sb egg-nucleus by + ^^ haplo-IV sperm, the second from + ^^ 

 egg-nucleus by haplo-IV Cy sperm. There must in fact have been 

 double fertiUzation of a binucleate egg. 



Mosaics due to somatic elimination of chromosomes are also found 

 in plants (Crepis, etc.). The most interesting type of plant mosaic 

 arises in a different way and is spoken of as a graft hybrid or plant 

 chimaera.^ If a graft is made of a shoot of one species on to a stock of 

 another, branches may be obtained which consist either of longitudinal 

 sectors of the two different species (sectorial chimaeras) or of a core of 



^ McClintock 1932. 2 Stem and Sekiguti 1931. 



^ Revs. Jones 1935, Weiss 1930, cf. j0rgensen and Crane 1927. 



