THE CHROMOSOMES IN HEREDITY. 237 



There is reason to believe that the division-products of a given 

 chromosome 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 re- 

 duced 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, chromo- 

 some a, from the mother in the presynaptic cells of the offspring 

 may be regarded as the physical bases of the antagonistic unit- 

 characters A and a of father and mother respectively. In syn- 

 apsis, copulation of the homologues gives rise to the bivalent 

 chromosome Aa, which as is indicated above would, in the re- 

 ducing division, be separated into the components 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, 



AS a $ 



A 9 9 



which would yield four combinations, 



AS + A 9 = AA ' 



AS + a 9 = Aa 



a S + A 9 = aA 



a S + 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- 

 nomena of germ-cell division and of heredity arc seen to liave the 

 same essential features, viz., purity of units (chromosomes, charac- 

 ters) and the independent transmission of the same ; while as a 

 corollary, it follows in each case that each of the two antago- 

 nistic units (chromosomes, characters) is contained by exactly 

 half the gametes produced. 



The observations which deal with characters have been made 



