ARTICLES 439 



the fertilised egg, one half of the chromosomes are of strictly 

 paternal origin, the other half of maternal." ^ 



This conclusion received strong support from the later work 

 of Riickert/ Hacker,' and others, who have been able to trace 

 and follow through the later cleavage stage of the egg the two 

 sets of chromosomes. 



Investigations on the cytology of hybrids has added a 

 further proof in support of the theory of chromosome indivi- 

 duality ; an excellent example of this is furnished by the 

 cytological work of Doncaster on hybrids of two species of 

 moths.* Nyssia zonaria has 112 small chromosomes as the 

 unreduced number, four of which are larger than the rest. 

 In the related species of moth, Biston hirtaria, there are 28 

 chromosomes, four of which are smaller than their fellows. 

 After the maturation division has taken place each species has 

 the reduced chromosome number, viz. Biston 14, two smaller 

 than the rest. Nyssia 56, two larger than the rest. 



The chromosomes of the hybrid are 70 in number, 14 of 

 which are noticeably larger than the rest. Perhaps the most 

 interesting part of all is, however, the subsequent behaviour 

 of these chromosomes during spermatogenesis. As each species 

 has only contributed a single set of chromosomes, it is impossible 

 for them all to pair together at synapsis, with the result that the 

 nuclei of the spermatocytes contain only a few less chromosomes 

 than do the spermatogonia! nuclei. 



Except on the theory of the individuality of the chromosomes 

 this fact, and others which might be cited, is inexplicable. 



Sex Chromosomes. — It has already been mentioned that the 

 chromosomes of a species are in general constant in number, and 

 that it is legitimate to conclude that half of them are of 

 paternal origin and half of maternal. During the development of 

 the germ-cells, however, this diploid number becomes reduced to 

 one half — the haploid number. The cell divisions by which this 

 is brought about are known as the maturation divisions, and are 

 comparable in both sexes ; the chief difference being that, in the 

 case of the male, four spermatozoa are formed by the division of 

 the spermatogonium, or mother-cell, but, in the case of the female 

 only one egg with its polar bodies, which ultimately disappear. 

 Broadly speaking, the polar bodies are the means by which half 

 the chromosomes are extruded from the maturating egg. 



This reduction occurs by a complicated method which cannot 

 be dealt with here, but briefly it consists in the pairing together 

 of the maternal and paternal chromosomes, which subsequently 



1 Boveri, Th., Merkel und Bonnet's Ergebnisse I., 1891, p. 410. 



2 Riickert, J., Anat. Anzeiger., 10, 1895. 



3 Hacker, V., Zool. Jahrb., 5, 1892. 



* Harrison, J. W., and Doncaster, L., Journ. genet., 3, 1914, p. 234. 



