2 NUCLEUS 237 



phenomenon whereby often more than a single property is regulated 

 from one locus of the chromosome. This view would also account for 

 minor distinctions in activity of homologous active regions in 

 different individuals and would explain polyallely. There must be some 

 correlation between the carrier and the active region, in the same way 

 as a co-enzyme can only develop its activity in close conjunction with 

 the apo-enzyme; to some such reciprocal action must be attributed 

 the different ways in which certain phaena are actualized. To be brief, 

 the carrier model serves to make intelligible most of the knowledge 

 acquired by research into heredity. 



There being good grounds for assuming that the leptonema has 

 a double structure, falling into two chromatids after conjugation, the 

 carrier discs may be represented as two halves, each with an operative 

 region of the same value (Fig. iz6e). Of these two, only one need 

 be struck by the rays for the origin of a mutated gamete, since the 

 chromatids are separated from each other in the formation of tetra- 

 cytes. 



The advantage which the carrier hypothesis possesses, as compared 

 to th.Q fibrillar hypothesis developed in the chapter on chromosomes, is 

 that it disregards the disputed question of molecular protein structure 

 and nucleic acid intercalation ; the elementary units may be conceived 

 of either as fibrillar protein units coiled in any way, or as globular 

 proteins. Then, the carrier hypotheses makes the gene and operative 

 region of suhmicroscopic dimensions, whereas the fibrillar hypothesis 

 allows the gene to be of amicroscopic size represented by side chain 

 groups of polypeptide chains. In the first edition this picture un- 

 warrantably simplified the exceedingly complicated facts. On the 

 other hand, the fibrillar hypothesis has to its credit the plausibility it 

 confers upon the shape, cleavability and self-duplication of the chro- 

 mosomes. It will therefore be the aim of research to reconcile these 

 two hypotheses to a concordant theory by endeavouring to fathom 

 the suhmicroscopic morphology of those proteins which represent 

 neither their extreme fibrillar, nor an independently dispersed globular 

 form. 



Function of the desoxyribose nucleic acids. The desoxyribose nucleic 

 acids, which were at first thought to be the hereditary substance par 

 excellence, are of relatively uniform chemical constitution and, in 

 their molecular morphology, lack the diversity required by genetics. 



