V NUMBER OF CHROMOSOMES 143 



In Ascaris canis (Walton, 19 18) the chromosomes in the somatic 

 cells fragment into two, so that these mitoses have double the number 

 of chromosomes present in the germ track. This seems to be a not 

 uncommon phenomenon. Thus the bisexual form of Ascaris nigrovenosus 

 (Schleip, 1912) has in the germ track eleven ( c^ ) or twelve ( ? ) chromo- 

 somes, the somatic tissues having double that number of much smaller 

 ones. Doncaster (1910) found one male of the gall-fly Neuroterus 

 lenticularis in which the number of chromosomes in the somatic tissues 

 was double that in the spermatogonia. In the bee [Apis mellifica — 

 Meves, 1907 ; Nachtsheim, 19 13) 2w is 32, judging from the meiotic 

 divisions, but the somatic cells often show 64. Armbruster (1913) found 

 also in the case of the soHtary apid Osmia cornuta that the number of 

 chromosomes is much higher in the mitoses of the soma than in those 

 of the germ track. 



Sometimes the process of fragmentation is less orderly, affecting 

 only certain cells, and these to varying degree, resulting in apparently 

 capricious variation of chromosome number. This has been studied by 

 Hance in the pig (1918 a) and in Oenothera (1918 h). 



In the pig the spermatogonial chromosomes always number 40. 

 In the ninety-one somatic cells in which the chromosomes were counted, 

 the number varied from 40 to 57. It will be noticed that none had less 

 than the type number. Most of them had more, only four having exactly 

 40. In order to obtain evidence as to whether the increase is due to 

 fragmentation or multiplication, Hance measured the sum of the lengths 

 of all the metaphase chromosomes in the spermatogonia and in the 

 somatic cells. In the former the sum of the lengths of all the chromo- 

 somes varied from ii8-6 to 177-6 units of measurement. The combined 

 lengths of all the chromosomes in the different somatic cells, in spite of 

 the variation in their number, fell within the same limits, with one 

 exception in which they totalled 117 units. This evidence, while not 

 conclusive in view of the variation of chromosome lengths in different 

 tissues, the possibility of regulations, and so on, is certainly greatly in 

 favour of the numerical increase having been caused by fragmentation 

 of the 40 " type " chromosomes. 



Closely similar results were obtained from Oenothera scintillans. In 

 this evening primrose 2w = 15, it being, like 0. lata (p. 146) one of the 

 Oenothera mutants which possesses an extra chromosome. In the somatic 

 tissues, however, while the number 15 is the commonest, it varies from 

 15 to 21. By measuring the lengths of aU the chromosomes and adding 

 them together, he found that the average total amount of chromatin 

 was approximately the same in nuclei with 15 chromosomes as in those 

 with 16, 17, 18, 19, 20 or 21. Hence he concludes that the higher numbers 

 have been derived from the lower by fragmentation of one or more of 



