i68 CYTOLOGY 



CHAP. 



avoided. Moreover, since the elements are differentiated, a mere mass 

 reduction — as, for instance, by diminished growth of the chromatin 

 between mitoses — would not meet the case. Weismann indeed long ago 

 postulated from theoretical considerations the occurrence of a reduction 

 such as takes place in gametogenesis. The process of meiosis has now 

 been universally recognized as pecuUarly adapted to bring about the 

 qualitative reduction required. This is especially the case since the dis- 

 covery that, of the two chromosomes which unite to form each bivalent, 

 the one is derived from the male and the other from the female parent. 

 The consequences of this for the processes of heredity are discussed under 

 the next section (D). The existence of meiosis, and the entire absence of 

 any discoverable analogous process for the cytoplasm, is such an obvious 

 argument in favour of the view that the whole of the idioplasm is con- 

 tained in the nucleus, that it is unnecessary to enlarge further upon it. 



D. THE PARALLEL WHICH EXISTS BETWEEN CHROMOSOME 

 BEHAVIOUR AND THE RESULTS OF BREEDING EX- 

 PERIMENTS. 



The results of Mendehan work find a ready interpretation in the dis- 

 coveries of cytologists, and this forms one of the most fascinating chapters 

 of Biology, which, however, can only be treated in brief fashion here. 



The fundamental step in establisliing the parallel — to caU it for the 

 moment by no more committal name — between breeding experiments 

 and cytological observations was taken by Sutton (1903), when he 

 pointed out that the nucleus contains a double series of differentiated 

 chromosomes, one series contributed from each parent, and that in 

 syndesis the chromosomes from the one parent pair with the homo- 

 logous chromosomes of the other. 



If we consider the simple Mendelian case, involving one character 

 only — say the feather colour in certain fowls (/I = black, a = " splashed 

 white " ; see Bateson, Saunders and Punnett on the Andalusian fowl)— 

 the results of crossing the two forms may be represented in the usual way: 



Zygotes AA x aa P 



black white 



Gametes A A \ a a 



I 

 Zygote Aa Fj 



grey 



Zygotes 1 AA black + 2 Aa grey + i aa white F, 



2 



