May 30, 1913] 



SCIENCE 



817 



this principle the improvement of domestic 

 races of plants and animals is being at- 

 tempted in many parts of the world to-day. 

 Now, it is obvious that we should be able 

 to understand the behavior of unit-char- 

 acters, at least in some degree, if it could 



Tertifixatlon 

 UnCon of! 



ABCD 



question how the splitting up and recom- 

 bination of particular groups of such char- 

 acters takes place. The main part played 

 by microscopical research has been to bring 

 forward proof that the hereditary characters 

 are somehow connected with separate bod- 



CteoYoge' 



J>upted(> arct^S 



JiBCl> a^cd. 



aBCd 



Somatic Divisions 

 Duplex Graufis 



AaBLCcDd. 

 jSynafrsij 



J^educteon Germ Cetis 



Diyision ^'implex 0rcupS 



be shown that they are somehow dependent 

 individually upon separate structural ele- 

 ments or different chemical substances that 

 may be separately transmitted through the 

 germ-cells. It is just this which micro- 

 scopical reseai'ch and experimental re- 

 searches on heredity, taken together, have 

 demonstrated. They have accomplished 

 more than this. They have not only shown 

 with a high degree of probability how the 

 transmission of unit-characters is effected, 

 but have thrown at least some light on the 



ies, contained in or formed from the cell- 

 nucleus, and known as the chromosomes. 

 Besides the chromosomes the cell also con- 

 tains another kind of bodies found in the 

 cell-protoplasm, and known as chondrio- 

 somes or plastosomes. These too are very 

 likely connected with heredity; but their 

 true significance has not yet become very 

 clear, and we shall hardly have time to con- 

 sider them within the limits of this address. 

 With the aid of the accompanying dia- 

 gram (Fig. 1) we may consider a few es- 



