THE ORIGIN OF VARIATION 



RIGHT ARM OF 



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CHROMOCENTER <&■■* '^^^ ' 





CHROMOSOMES FROM OVARIAN TISSUE 

 (Same enlarqement) 



LEFT ARM OF 

 CHROMOSOME 



Figure 77. The Salivary Gland Chromosomes of DrosopJiila. (From Altenburg, 

 "Genetics," 2nd Ed., Henry Holt & Co., Inc., 1957.) 



level appears to be involved in the great size inerease. These ehromosomes 

 have a eross-banded appearanee, and the order, shape, and intensity of 

 the bands are all perfeetly regular, so that the salivary gland ehromosomes 

 ean be mapped, and even small segments ean be identified by reference 

 to a standard map. 



ARCHITECTURAL CHANGES IN THE CHROMOSOMES 



The above data lead to the conclusion that, beneath the homogeneous 

 appearance so often presented by the chromosomes, there is a longitudinal 

 physicochemical differentiation, the "architecture of the chromosomes." 



216 



