436 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1930 



chromosome made up of two shaded halves (1-1) (pi. 5 6'). This 

 type gives us information regarding the factors in the shaded (or 

 1-1) half of the white chromosome. 



A type with the extra chromosome like the other two in the set is 

 called a primm'y type in distinction from secondary types, in which 



JT/^ND/IQD LINE 1. 



J 



Figure '.i. — Diagram of cliromosomes in standard line 1 



the extra chromosomes have been formed apparently by a breaking 

 into two of a normal chromosome and a joining together of like 

 halves. In the models we have not shown how the chromosomes in 

 the trisome, or set of three, are attached together in the reduction 

 divisions. Plate 5 A, B, C shows the appearance under the micro- 

 scope of the chromosomes of the primary and the two secondaries of 



