234 FINE-STRUCTURE OF PROTOPLASM II 



where small active groups are similarly carried by a larger protein 

 complex system (see Fig. 119, p. 208). I therefore propose to discuss 

 the picture of carrier and prosthetic region for the genes as well and 

 to call this view the ^''Carrier hypothesis^'' . 



Since it is not the salivary gland chromonemata stretched to the 

 utmost, with their hypothetical submicroscopic thickness, which 

 operate in hereditary processes, but the considerably shorter meiotic 

 chromosome threads (leptonema, zygonema), the size of the gene 

 should be derived from the conditions produced by reduction division. 

 We learn from genetics that the regions inciting mutation are placed 

 linearly in the conjugating chromatids; consequently the target areas 

 must likewise be aligned lengthwise in the leptotenic chromosomes. 

 As the x-chromosome is supposed to contain about 1800 genes 

 (Timofeeff-Ressovsky, 1940), all sensitive regions of 4 m^^ diameter 

 should together produce a length of 7.2/^. Bearing in mind that, 

 according to Muller (1935), the genetically active volume of the 

 x-chromosome is roughly ^12 Z*^? one finds for the thickness x of the 

 extended leptotenic chromosome prior to conjugation of the chro- 

 mosomes, ^l^-K^-Tz-j.z = 7i2) from which x = 0.12 /^ is derived for 

 the thickness of the so-called leptonema. This value may be of the 

 right order of magnitude, since the diameter of the leptonema is in 

 the vicinity of microscopic resolving power. 



If the genetically active chromomeres of the leptonema are now 

 divided up into submicroscopic slices of the thickness of a target area, 

 the region corresponding to a gene should be included. In this view 

 and by this calculation, the gene should be a flat disc having an 

 estimated diameter of 120 mju and thickness of 4 m^. We are com- 

 pletely ignorant as to where the target areas lie in these discs: the 

 arrangement may be any of an almost unlimited variety, as in Fig. 1 26 c. 

 The only certainty we have is that, given the linear ahgnment of the 

 loci of the mutations, they must be juxtaposed in the axis of the 

 leptonema. In Fig. i26e such a gene disc is represented on the same 

 scale as the dimensions of the gene calculated by Muller (1935). The 

 position of the target area within the gene being unknown, it is shown 

 as a globule placed arbitrarily anywhere in the disc. It is interesting to 

 note that this size of the gene tallies well with that computed by Muller 

 (chromonema cross-section x lengthof gene), although found by total- 

 ly different means (leptonema cross-section x diameter of target area) : 



