XIX 



HEREDITY 



377 



trate, following Prof. Punnett, what may be called 

 Mendel's theory as distinguished from his rule. In the 

 black fowls, every germ-cell, whether ovum or spermato- 

 zoon, is supposed to bear a representative factor for 

 blackness. The black birds are " homozygous ' as 

 regards blackness. Similarly the germ-cells or gametes 

 of the white fowls bear a representative factor for white- 

 ness and none for blackness. When a "black* egg-cell 

 is fertilised by a " black " spermatozoon, the result will 

 be a black bird. But when a "black' egg-cell is fer- 

 tilised by a " white " spermatozoon, or a " white " egg- 

 cell by a " black ' spermatozoon, the resulting zygote 

 (the fertilised egg-cell) will contain the representatives 

 or factors or genes for both blackness and \vhiteness, 

 and will develop into a blue bird, which is technically 

 ' heterozygous ' as regards blackness and w r hiteness. 



Now what- Mendel shrewdly suggested was that in the 

 history of the germ-cells within the hybrid (F^ form there 

 is a segregation of the contrasted factors, so that half the 

 germ-cells have the one and half have the other. Half of 

 the ova have the dominant factor and half the recessive 

 factor because of this segregation into two contingents. 

 Half of the spermatozoa similarly carry the dominant 

 factor and half the recessive factor. Then, if fertilisation 



SPERM-CELLS. 



EGG-CELLS. 



1DD 



FIG. 117. DIAGRAM ILLUSTRATING MENDELISM. 



