344 INTRODUCTION TO CYTOLOGY 



after grafting or decapitation, notably in Solanacese.^" Another interest- 

 ing condition is seen in certain gynandromorphs, where portions of the 

 body differing in chromosome number differ also in sex (p. 381). In 

 certain Diptera it has been observed that tracheal cells, intestinal cells, 

 or rectal gland cells may become polyploid while the germ cells retain 

 the normal number.-^ 



How Differences in Number Arise. — Sudden alterations in chromo- 

 some number are brought about by aberrations of mitosis, by the union of 

 unusual or unlike complements in syngamy, and by a combination of these 

 two processes. In a number of instances such aberrations have been 

 directly observed in progress, while in certain others it can reasonably be 

 inferred that they are responsible for differences in number found in the 

 field or breeding plot. Moreover, some of them may be induced experi- 

 mentally, as by means of heat or X-rays. ^^ Some of the more common 

 phenomena leading to numerical changes are listed below. 



JJ.^ JL. ^..^^ 



? <? ? 



Fig. 196. — Chromosome complements of Drosophila melanogaster as they appear 

 during mitosis in a female, a male, and a non-disjunctional female with an extra X-chromo- 

 some. {After Morgan.) 



Two chromosomes may fail to separate in the anaphase of mitosis 

 and pass together to one pole. In meiosis this non-disjunction results in 

 the formation of some spores or gametes lacking one chromosome (n — 1) 

 and others with both members of the non-disjoined pair (n + 1). Unions 

 of gametes after this aberration may give rise to new individuals with 

 2n — 2, 2n — 1, 2n, 2n + 1, or 2?i + 2 chromosomes (Fig. 196). Gam- 

 etes and zygotes with extra chromosomes are viable more often than those 

 lacking chromosomes, so that hyperploid types arise more frequently than 

 hypoploid ones. Sometimes more than one chromosome pair may fail to 

 disjoin, giving still greater departures from the diploid number in the 

 offspring. When the halves of a split chromosome in somatic tissue pass 

 to the same pole instead of separating, hyperploid and hypoploid nuclei 

 result. Such an aberration may have an immediate visible effect upon 



20 Winkler (1916), J0rgensen (1928), F. Sansome (1929), Lindstrom and Koos 

 (1931). 



21 Holt (1917; on Culex), Frolowa (1926, 1929; on Drosophila and other genera). 



22 For literature, see footnotes in Chapter XIII. 



