32 



twenty-two elements ^^ which can be 

 seen to make up two series like that of 

 the mature germ-cells, and hence may 

 be designated as A, B, C . . . K -h A, 

 B, C . . . K. Synapsis results in the 

 union of homologues and the produc- 

 tion of a single series of double-ele- 

 ments thus: AA, BB, CC . . . KK, 

 and the reducing division affects the 

 separation of these pairs so that one 

 member of each passes to each of the 

 resulting germ-products. 



There is reason to believe that the 

 division-products of a given chromo- 

 some in Brachystola maintain in their 

 respective series the same size relation 

 as did the parent element; and this, 

 taken together with the evidence that 

 the various chromosomes of the series 

 represent distinctive potentialities, 

 make it probable that a given size- 

 relation is characteristic of the physical 

 basis of a definite set of characters. 

 But each chromosome of any reduced 

 series in the species has a homologue in 

 any other series, and from the above 

 consideration it should follow that 

 these homologues cover the same field 

 in development. If this be the case 

 chromosome A from the father and its 

 homologue, chromosome a, from the 

 mother in the presynaptic cells of the 

 offspring may be regarded as the phys- 

 ical bases of the antagonistic unit-char- 

 acters A and a of father and mother 

 respectively. In synapsis, copulation 

 of the homologues gives rise to the 

 bivalent chromosome Aa, which as is 

 indicated above would, in the reducing 

 division, be separated into the com- 

 ponents A and a. These would in all 

 cases pass to different germ-products 

 and hence in a monoecious form we 

 should have four sorts of gametes. 



A$ 

 A9 



a9 



SUTTON 



which would yield four combinations, 



As + A9 = AA 

 As + a 9 = Aa 



as + A9 = aA 



aS + a 9 = aa 



Since the second and third of these are 

 alike the result would be expressed by 

 the formula AA : 2Aa : aa which is the 

 same as that given for any character 

 in a Mendelian case. Thus the phe- 

 ?iomejia of germ-cell divisio?i and of 

 heredity are see?i to have the same 

 essential features, viz., purity of units 

 {chromosomes, characters) and the in- 

 depeiident transijussion of the saine; 

 while as a corollary, it follows in each 

 case that each of the two antagonistic 

 units (chromosomes, characters) is 

 contained by exactly half the gametes 

 produced. 



The observations which deal with 

 characters have been made chiefly 

 upon hybrids, while the cytological 

 data are the result of study of a pure- 

 bred form; but the correlation of the 

 two is justified by the observation of 

 Cannon ^- that the maturation mitoses 

 of fertile hybrids are normal. This 

 being the case it is necessary to con- 

 clude, as Cannon has already pointed 

 out, that the course of variations in 

 hybrids either is a result of normal 

 maturation processes or is entirely in- 

 dependent of the nature of those divi- 

 sions. If we conclude from the evi- 

 dence already given that the double 

 basis of hybrid characters is to be 

 found in the pairs of homologous chro- 

 mosomes of the presynaptic germ- 

 cells, then we must also conclude that 

 in pure-bred forms likewise, the paired 

 arrangement of the chromosomes indi- 

 cates a dual basis for each character. 

 In a hypothetical species breeding ab- 

 solutely true, therefore, all the chro- 

 mosomes or subdivisions of chromo- 



11 Disregarding the accessory chromosome 

 which takes no part in synapsis. 



12 Cannon, W. A., loc. cit. 



