The Cellular Basis 



181 



is fertilized by a sperm which lacks the X chromosome a male is 

 produced, if fertilized by the other type a female results. This 

 correlation between the presence or absence of a whole chromo- 

 some and of a developed character such as sex, was the first case 

 of the kind that was known and more than anything else it 

 served to prove that the chromosomes contain the Mendelian 

 factors. The absence of a whole chromosome is plainly visible 

 under the microscope, whereas the absence of a single factor or 

 gene from a chromosome would never have been seen. These 

 fortunate cases in which the male lacks a whole chromosome seem 

 almost to have been intended to give ocular proof of the chromo- 

 somal theory of heredity; they are to biology what the rings of 

 Saturn are to astronomy, a visible confirmation of a great 

 theory. 



Bynaprii 



AbeD 

 ** Oenn Cell. 



Dii-iiion Simple* Group, 



Somatic Division* 

 Duple* Group* 



FIG. 60. DIAGRAM OF GERM CELLS CORRESPONDING TO FIG. 59, show- 

 ing the union of maternal chromosomes (ABCD) and paternal ones 

 (abed) in fertilization, their distribution in cleavage, their union into 4 

 pairs (Aa, Bb, Cc, Dd) in synapsis and the separation of the pairs in the 

 reduction division. Only 2 of the 16 possible types of germ cells are shown 

 at the lower right. (After Wilson.) 



