V NUMBER OF CHROMOSOMES 151 



While 0. gigas is probably a true tetraploid form, a case of chromo- 

 some doubling has been described by Farmer and Digby (1914), in which 

 it appears probable that the doubling was produced by fragmentation. 

 Primula florihunda crossed by P. verticillata gave a hybrid of a distinct 

 type known as P. kewensis. In both parents 2w = i8, and this was also 

 the number in the hybrid. The hybrid plant produced only " thrum " 

 flowers, and was therefore self-sterile. It was reproduced vegetatively 

 by cuttings, and eventually a single " pin " flower appeared, which 

 allowed of fertihzation by one of the thrum flowers, and from this a 

 fertile race of P. kewensis was obtained. This race was found, however, 

 to have double the number of chromosomes present in the original P. 

 kewensis, 2n being 36. The chromosomes are also smaller in the fertile 

 race, measurements giving the following results : 



Mean volume of single Total volume of all 



Chromosomes. Chromosomes. 



Race with 18 chromosomes -8141 " i4"65 



Race with 36 chromosomes -4088 i4'7i 



These measurements clearly suggest that in this case the doubling 

 of the chromosome number has been brought about by transverse 

 fragmentation, and not by chromosome fission unfollowed by nuclear 

 division. A true tetraploid Primula has, however, been found in another 

 case, as described in the next chapter (p. 170). 



The last cause of variation in chromosome number which we have 

 to consider is fusion of chromosomes. An example of this has already 

 been given in the case of Culex pipiens (p. 126). It is obvious that here 

 we are dealing with an apparent variation only, since all the chromosomes 

 are present, though they may be indistinguishably fused in pairs. 



A Sycon sponge (Jorgensen, igio a) probably presents a case similar 

 to Culex. In this sponge the number of chromosomes in the gamete is 

 eight, and in accordance with this sixteen chromosomes are found in the 

 metaphase of the first cleavage division. In all other cells examined, 

 however (mesoderm cells, oogonia, oocytes), the number is always eight, 

 except in the early prophases, when it is more, though the number could 

 not be exactly counted. Jorgensen brings forward evidence to show that 

 a fusion of pairs of homologous chromosomes takes place in the telophase 

 of the first cleavage division. 



A particular class of cases of the halving of the chromosome number 

 falls to be mentioned here. A union of the chromosomes^now in the 

 haploid number — in pairs sometimes takes place in the anaphase of the 

 first meiotic division or between the two divisions. This has been de- 

 scribed in Birds {Numidia and Callus — Guyer, 1909, 1916 ; Columba — 

 Smith, 1913), Mammals (Man — Guyer, 1910 ; Didelphys -Jordan, 1912) ; 



