THE GENETIC NATURE OF TAXONOMIC DIFFERENCES 255 



occurs in somatic tissue, the two daughter nuclei may unite and con- 

 stitute a single nucleus with the tetraploid chromosome number. If a 

 shoot originates from such a cell, it will also be tetraploid, and bear 

 flowers which give diploid gametes (if the reduction is regular, cf . 

 p. 69). Failure of cell-division may occur in germinal tissue shortiy 

 before the reduction division, in which case diploid gametes will be 

 formed immediately in the descendants of the affected cell; this process 

 is known as sjoidiploidy. The meiotic divisions themselves may also 

 fail, either by an imperfect separation of the daughter nuclei, which 

 gives a so-called "restitution nucleus," or by a failure of zygotene 

 pairing accompanied by a suppression of the second division; the latter 

 abnormaHty occurs particularly in hybrids. Both these types of failure 

 of meiosis give rise to diploid gametes, which will give tetraploids on 

 selling or triploids when crossed with normals. Similar processes occur 

 in the formation of unreduced gametes in organisms with diploid 

 parthenogenesis (p. 63). 



These processes of doubling were imtil recentiy not under experi- 

 mental control. In mosses^ it is possible to suppress one of the meiotic 

 divisions by the use of narcotics such as chloral hydrate, and thus to 

 obtain diploid gametophytes; recentiy Blakeslee^ has found that in the 

 higher plants anaphase separation in mitosis can be rather regularly 

 inhibited, with the formation of tetraploid restitution nuclei, by treat- 

 ment with colchicine. Failure of somatic mitosis can sometimes be 

 stimulated by various sorts of ill-treatment (e.g. extreme temperatures) 

 and processes of this kind may turn out to be appUcable to animals. 

 A failure of mitosis is rather common in the callus tissue growing over 

 wounds in Solanaceae,^ and many tetraploids have been produced from 

 shoots originating in such tissue. A special case of polyploid formation 

 occurs in some diplohaplonts, in which portions of the diploid sporo- 

 phytes may grow into plants which are gametophytes, although they 

 retain the diploid number."* 



Polyploids originating within a single species usually differ rather 

 Httie in appearance from the original diploids, although they are often 

 somewhat larger and more vigorous. Their fertility is usually lower 

 because of the irregularity with which the reduction takes place (p. 70). 

 More important for the study of natural polyploids are those which 



1 Marchal and Marchal 191 1. Rev. efifects of temperature etc. Sax 1937. 



2 Cf. Blakesiee and Avery 1937, Nebel and Ruttle 1938. For an attempt to 

 produce tetraploid animals in this way, see Pincus and Waddington 1939. 



3 Winkler 1916, Jorgensen 1928. * Wettstein 1927. 



