LECTURE VII. 

 THEORIES OF HEREDITY. 



The cell regarded as the unit in phenomena of life. 

 Reasons why the cell may not be the life unit. Cells 

 forced to form organs. Organs built up of cells. 



The sexual cell specialized to carry all the qualities 

 of the parent, both latent and developed. 



I. Theories of Encasement and Evolution. 



II. Theory of Epogenesis ; building up of structure ; 

 one cell upon another. 



III. Darwin's theory of Pangenesis. Each cell send- 

 ing out from itself to every part of the body gem mules 

 or minute buds ; u these inconceivably minute and 

 numerous as the stars in heaven." Each of these cap- 

 able of reproducing the qualities of the cell it came 

 from. 



IV. Brooks. Differentiation of sex in gemmules. 

 The male gemmules active and varying, the female 

 sessile and conservative. 



V. Weismann's theory of continuity of germ plasm 

 from generation to generation. Germ cells and so- 

 matic cells. Nucleus the directive part of the cell ; 

 virtually unchanged from generation to generation ex- 

 cept by forces within it. External changes of the 

 xoma do not reach it. 



