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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 5 



with like and, as shown in the smaller diagram above, figure 7, this 

 is made possible by the end of one component turning back on itself. 

 In figure 8 is drawn a short region of chromosome 2. One com- 

 ponent has a deficiency for certain genes; the opposite normal chro- 

 mosome forms a bulge in the region of the deficiency, allowing like 

 bands to come together above and below the deficiency level, 



THE PHYSIOLOGICAL PROPERTIES OF THE GENES 



If, as is generally implied in genetic work (although not often 

 explicitly stated), all the genes are active all the time, and if the 



Diagram ofright half of 

 Chromosome 3 



Terminal Inversion "Ca-laa" 

 Heterozygote 



Salivary chromosome 



34d4 (REviseo asae) cas 



FiGUKB 7. — Salivary gland preparation of right arm of the third chromosome, illustrating 

 a terminal inversion in one of the two components. The two components are fused 

 together throughout part of their length and are separate in other parts as shown 

 in the small diagram above and to right. The terminal inversion conjugates with 

 the noninverted end by turning back, as proved by the sequence of the bands. (After 

 Bridges.) 



characters of the individual are determined by the genes, then why 

 are not all the cells of the body exactly alike ? 



The same paradox appears when we turn to the development of 

 the egg into an embryo. The egg appears to be an unspecialized cell, 

 destined to undergo a prescribed and known series of changes 

 leading to the differentiation of organs and tissues. At every di- 

 vision of the egg the chromosomes split lengthwise into exactly 

 equivalent halves. Every cell comes to contain the same kinds of 

 genes. Why, then, is it that some cells become muscle cells, some 

 nerve cells, and others remain reproductive cells? 



The answer to these questions seemed relatively simple at the end 

 of the last century. The protoplasm of the egg is visibly different 



