6 Mohr. 



This differeuce in the form ol the eggs could not be explained 

 as due merely to au incomplete development, since eggs in the ovaries 

 of not-singed females are elongated and have typicallj^ long and slender 

 filaments near the anterior end at stages which undoubtedly are younger 

 than that of the eggs laid by the singed females. 



The fact that these deformed filaments stick to the surface of the 

 egg, in connection with their shifted position which prevents their 

 ends from projecting beyond the anterior end of the egg, explains why 

 they were at first overlooked. 



Repeated tests proved that the defective condition of the eggs 

 was constant in the singed females and that the eggs without exception 



Fig. 2. 



1. Eggs laid by wild -type females. 2. Eggs from the ovary of wild -type females. 



3. Eggs laid by singed females. 4. Eggs from the ovary of singed females. 



failed to develop. During the experiments the singed males and the 

 heterozygous females from which the sterile singed females were derived 

 were outcrossed repeatedly, so it does not seem likely that the egg 

 alteration and the sterility, is due to a special gene linked to the 

 singed. 



Singed is the first sex-linked mutation in Drosophila which causes 

 a defective condition of the eggs with complete sterility. The sex-linked 

 gene fused was thought to cause complete sterility of the homozygous 

 females until Miss Lynch showed ('19) that in rare cases fused females 

 give offspring with not fused males. The sex-linked recessive lozenge 

 was also believed to produce sterility of the females, but the author 

 has bred lozenge in unselected mass cultures without great difficulty. 



