64 AN INTRODUCTION TO MODERN GENETICS 



(2) Failure of the spindle mechanisnij although pairing is regular, 

 occurs in plants, e.g. Hieracium. The first division comes to an end at 

 the diakinesis stage and a spindle is not formed; the diploid nucleus 

 resulting is said to be a restitution nucleus. A second division follows 

 in which the bivalents divide, so that two diploid gametes are formed. 

 In animals the failure may affect the second division, which is either 

 omitted altogether or gives rise to a second polar body which then 

 fuses again with the egg-nucleus. A more refined failure of the spindle 

 mechanism is probably responsible for the differential elimination of 

 one chromosome in the production of males in diploid parthenogenesis 

 of Aphids. Failures of the spindle may also occur in mitosis, either 

 regularly as in the first cleavage division of some haploid partheno- 

 genetic eggs, or sporadically. It leads to a doubling of the chromosome 

 number. In the eggs this merely restores the normal diploid number; 

 but if a failure occurs in germinal cells shortly before meiosis it leads 

 to the formation of tetraploid cells which on reduction give diploid 

 gametes. The phenomenon is known as syndiploidy and provides 

 another mechanism by which occasional diploid gametes may be 

 formed. 



A failure of the spindle in meiosis has been provoked artificially by 

 abnormal temperatures and narcotics (see p. 255). A similar failure in 

 mitosis has been brought about in the cleavage divisions of echinoderms 

 by shaking and in plants by various treatments. In a few plants, par- 

 ticularly Solanaceae, a fairly high proportion of tetraploid cells occurs 

 in the callus tissue which grows over wound surfaces, and these cells 

 may grow into tetraploid shoots. 



C. THE THEORY OF POLYPLOIDS^ 



The normal diploid organism contains two of each kind of chromo- 

 some. Chromosome complements are also foimd, particularly in plants, 

 in which each chromosome is represented three, four, or more times. 

 Such organisms are spoken of as polyploids, and we may find polyploid 

 series of related plants in which the chromosome numbers form an 

 arithmetical progression. Thus there are different species of the genus 

 Solanum with the chromosome numbers 24, 36, 48, 60, 72, 96, 108, 

 120, and 144, all the numbers being multiples of 12. This can be taken 

 as the basic number of the series, the members of which can be repre- 

 sented as 2x diploid, 2>x triploid, ^x tetraploid, 5^ pentaploid, 6x hexa- 



^ General references: Darlington 1937, chap. 6, Miintzing 1936, Sansome 

 and Philp 1932 



