THE METHODS OF EXPERIMENTAL EMBRYOLOGY 483 



The Methods of Experimental Embryology 



Experimental breeding is usually carried on under conditions 

 that are most favorable for the organisms concerned. The results 

 obtained are, therefore, comparable with what might be expected 

 to occur in uncontrolled breeding in the natural habitat. This 



Zygotes from 

 which Pj 

 developed 



Gametes of P, 



Zygotes of Fj 



Gametes of F, 



Zygotes of F~ 



Red-eyed 

 female 



White -eyed 

 female 



Red-eyed 

 male 



White-eyed 

 male 



Fig. 258. — Diagram illustrating independent segregation and possible recom- 

 binations of X-chromosomes carrying genes determining eye-color, in a cross 

 between a white-eyed female Drosophila and a red-eyed male (cf. Fig. 251, 

 p. 473). This is the reciprocal of the cross shown in Fig. 257, and the symbols 



are described there. 



" normal " course of inheritance can be observed and interpreted. 

 However, in this as in many fields of study, a knowledge of what 

 happens under abnormal or unusual conditions may clarify 

 our understanding of the normal situation. 



In the examples cited to illustrate Mendehan principles, a 



