THE CHROMOSOMES 77 



some; for, the niiniber of chromosomes is hmited 

 while the nmiiber of independently inherited char- 

 acters is large. In Drosophila melanogaster it has 

 been demonstrated not only that there are exactly 



r 





Fig. 32. — Diagram showing the segmentation 

 of an egg into two, four, eight cells, etc. The 

 cells become arranged over the surface of a 

 sphere whose interior is filled with fluid. (After 

 Selenka.) 



as many groups of characters that are inherited to- 

 gether as there are pairs of chromosomes, but even 

 that it is possible to locate the hereditary elements 

 in particular chromosomes and to state the relative 

 position there of the factors for the characters. If 



