THE STRUCTURE OF THE CHROMOSOMES 141 



interpreted as a series of aspects observable when the matrix swells and 

 loses its chromaticity in the telophase. Certain cases of "telophase 

 splitting by median alveolation" formerly reported are interpreted on 

 similar grounds. The chromatic structure first appearing in the telophase 

 chromosomes is not actually produced at that time by alveolation or any 

 other process but is simply rendered visible by an alteration in the 

 chromaticity of the matrix. 



The Chromomere Theory. — Many years ago the theory was pro- 

 pounded that the small chromatic lumps often visible in the reticulum are 

 arranged in a linear series in a slender thread during the prophase and by 

 their division initiate its splitting. ^^ It was further supposed by some 

 workers that these small bodies, or chromomeres, are composed of still 

 smaller "chromioles," which are supported in the achromatic substance 



■4---i 14 ^ 4 t ff\ 



h ^ d f h 



/ 



*-^ X \ 



\^ 



/ T h i ^ "V- 



Fig. 79. — Chromosome B (synapsed pair) from spermatocytes of 13 individuals of 

 Phrynotettix magnus, showing constancy in size and arrangement of principal chromomeres. 

 The same constancy is shown in different cells of a single individual. {After Wenrich, 

 1916.) 



of the reticulum. The general opinion that such chromatic bodies are 

 more or less persistent nuclear units multiplying regularly in mitotic 

 division harmonized w^ell with the theory of heredity propounded by 

 Roux and Weismann, according to which the chromatic matter is quali- 

 tatively unlike in different regions of the nucleus, the arrangement of this 

 matter in the form of a long thread prior to its splitting being a means 

 whereby all the qualities, and hence hereditary potencies, are divided 

 and distributed to the daughter nuclei. 



Since 1900, chromomeres have been studied chiefly in animal sperma- 

 tocytes and the microsporocytes of plants. In the spermatocytes of 

 insects, especially grasshoppers, the chromomeres appear with great 

 clearness.^* In some examples studied with particular care, it has been 

 found that in the early prophase the chromomeres not only differ in size, 

 as claimed long ago by van Beneden, but are arranged in a characteristic 



" Balbiani (1876, 1881), Pfitzner (1881), van Beneden (1883). 

 1* McClung (1905 et seq.), Carothers (1917), Pinney (1908), Robertson (1915, 

 1916), Wenrich (1916), Janssens (1924). 



