220 



THE THEORY OF THE GENE 



somes. Their presence has been detected, in such cases, by 

 their occasional separation, as in Ascaris (Fig. 126), 

 from their attachment, or by the differential staining 

 properties of the X-chromosome in the male, or, as in 

 certain moths studied by Seiler, by the regular separation 

 of the compound chromosome into its components in the 

 somatic cells of the embryo. 



o^ 4,. 



Ooqon ia 



)erma 



onia 



Fig. 127. 



Diagram illustrating the distribution of the attached X-chromo- 

 somes in the male and female of Ascaris. (After Boveri.) 



9 



cf 



The attachment of the sex-chromosomes to ordinary 

 chromosomes, or autosomes, as they are called, involves 

 the mechanism of sex-linked inheritance, more particu- 

 larly should crossing-over take place in the male between 

 the autosome attached to the X and its mate lacking the 

 attached X. An example will illustrate the point at issue. 

 In Fig. 127 the X-chromosomes of Ascaris are indicated 

 by the black ends of those chromosomes to which they are 

 attached. In the female there are two X-chromosomes, 



