I 1 



lOO MENDELISM chap. 



dominant character is' carried by the mother, while 

 in the latter it is carried by the father. To bring 



the two cases into 

 © ^ + line it is necessary to 



suppose that the male 



Drosophila is always 



' ' heterozygous both for 



the sex-factor and for 



I 1 1 1 the factor for red-eye. 



(^ i ? ? ^2 The hypothesis was 



Fig. 28. confirmed by the re- 



Scheme illustrating the result of crossing a red- SUltS Obtamed whcn 

 eyed male Z)TO.vOii/ii/a with a white-eyed Jjj rprl-pvfd malpo 

 female. Sex signs as in Fig. 27. WUQ rCQ-eyea maiCS 



were mated with 

 white -eyed females. In all cases they gave red- 

 eyed daughters and white-eyed sons only. 



The brilliant researches of Morgan and his co- 

 workers in America have carried the analysis of sex 

 in Drosophila a stage further. They have been able 

 to correlate these peculiar differences in sex-heredity 

 with differences in the minute structure of the cells 

 of which the bodies of the two sexes are composed. 

 The microscopic cells of which the bodies of animals 

 and plants are built up are composed of a semi- 

 transparent jelly-like stuff called protoplasm. The 

 precise nature of this differs in different ce^ls : muscle 

 cells differ from liver cells, and these again from brain 

 cells. But all living cells agree in possessing a 

 definite structure, the nucleus, which lies inside the 

 cell, and apparently governs its activities. The 

 nucleus, generally a minute round or oval body, 

 possesses a definite structure of its own. Its most 

 characteristic components are the chromosomes — 



