xm] HAPLOID AND TETRAPLOID NUMBERS 193 



Again, the persistence of the reduced number of chromo- 

 somes in the cells of parthenogenetic embryos is an example, 

 of a somewhat different kind, of the retention of an abnor- 

 mal number when it has once been produced. In this case, 

 however, in some animals at least the normal (diploid) 

 number is restored in some of the tissues, probably by a 

 division of the chromosomes without nuclear division. In 

 some male Hymenoptera, for example, the haploid number 

 persists in the germ-cells, and, at least to a late larval stage, 

 in the cells of the nervous system, while in other somatic 

 cells the diploid number is restored. 



Instances of abnormality in chromosome number in the 

 opposite direction are afforded by so-called "tetraploid" 

 races of animals and especially plants, in which all the 

 nuclei contain twice as many chromosomes as are charac- 

 teristic of the species. In a number of plants (Primula, 

 Oenothera, etc.) races are known in which the chromosomes 

 have double the normal number ; frequently such plants are 

 giants, in consequence of the large size of their cells, and 

 often also they are apogamous, that is, their ovules develop 

 parthenogenetically without requiring fertilisation. This 

 apogamy of tetraploid plants may be compared with the 

 condition already referred to in Artemia, in which the race 

 with 84 chromosomes is parthenogenetic, while that with 42 

 reproduces sexually. In the banana three races have 8, 16 

 and 24 chromosomes respectively in their nuclei, and in 

 different species of Chrysanthemum the (haploid) numbers 

 9, 1 8, 27, 36 and 45 have been recorded 1 . WINGE points out 

 that in plants 6, 8, 9, 12 and 16 are much the commonest 

 haploid chromosome numbers, and that the next most fre- 

 quent numbers are simple multiples of these. All these facts 

 suggest that the chromosomes are to some extent individuals, 



. 1 TAHARA and TISCHLER, quoted by WINCE (1917). 

 D. c. 13 



