144 INTRODUCTION TO CYTOLOGY 



what extent the matrix shares in determining the form of the chromosome 

 aside from giving it a smoother contour. 



Chromosome Structure and the Gene Theory. — In later chapters it 

 will be shown why the chromosomes are thought to contain a series of 

 persistent units, or genes, which affect profoundly the course of develop- 

 ment in successive generations and thus represent causal agents in hered- 

 ity. At this point, those who are familiar with the gene theory may 

 consider certain points of a hypothesis which attempts to relate the 

 genes to the visible differentiations in the chromosome. ^^ 



As suggested by Alexander and Bridges, the fundamental constituent 

 of the chromosome is the "gene string," a linear aggregate of genes held 

 together primarily by their own attractive forces. The exact nature of 

 the genes is unknown, but they may be thought of provisionally as minute 

 bits of substances growing by autocatalysis and synthesizing a number of 

 materials including basichromatin, the most conspicuous of such mate- 

 rials. In the early prophase each gene is surrounded by its own small 

 mass of basichromatic matter; this is a chromomere. As the prophase 

 advances, some chromomeres increase in size more rapidly than others 

 because of a greater activity on the part of their genes, and they sooner 

 or later fuse to form the coarser and more uniformly chromatic chro- 

 monema. Hence larger lumps in this thread indicate groups of more 

 active genes. As the thread becomes thicker, the basichromatin just 

 beneath its limiting membrane is transformed into oxychromatin; this is 

 the chromosome matrix surrounding the still basichromatic chromonema 

 with its gene string. Because of its liquid nature the oxychromatic 

 matrix tends to round up and thus shorten the chromosome as a whole in 

 the later prophase, the relatively firm chromonema then being thrown 

 into coils. The matrix now becomes basichromatic again. In the 

 telophase the basichromatin decreases in amount and may largely or 

 completely disappear by the time the metabolic reticulum is fully devel- 

 oped; in the next prophase it is elaborated anew by the genes. This 

 discontinuity of the basichromatin is taken to mean that "chromatin" 

 is not the important "hereditary substance" but only a by-product of the 

 autocatalytic or reproductive activity of the persistent genes. Belling 

 calls the chromatic matter about each gene "gene chromatin" and the 

 abundant substance present later "extra chromatin." In certain cases 

 he observes at the center of each chromomere a minute dot which he 

 regards as "either a bare gene, or close to one" (1931a, p. 156).^^ 



18 Alexander and Bridges (1928). See also Belling (19286c, 1931a) and Reuter 

 (1930). 



1^ Certain provisional calculations of the possible size of genes place them well 

 below the limit of visibility with the ordinary microscope and suggest that they are 

 of the same order of magnitude as certain large protein molecules (see Morgan, 1922, 

 and Gowen and Gay, 1933). Recent evidence on the spacing of genes in the chromo- 

 some (p. 323) offers a basis for new speculations on this subject. 



