HEREDITY IN THE PROTOZOA 283 



missing parts. Should one end happen to have 

 some local peculiarity or abnormality, this may be 

 ''inherited" through that half, but not by the other 

 half. The abnormality may be said to be persistent 

 rather than to be inherited. 



Fission of protozoa is generally compared with 

 cell-division in the soma of higher forms. It is 

 known that here the two daughter-cells receive the 

 chromosome complex of the mother cell and usually 

 like parts of the maternal cytoplasm. When, as 

 often occurs in early embryonic cells (blastomeres) , 

 the two daughter-cells get very different proportions 

 of the visible inclusions of the original parent cell 

 (such as pigment, or yolk) it has been shown by 

 centrifuging experiments that such inclusions do not 

 have a differentiating influence on the cells that 

 contain them. Embrj^onic differentiation might 

 however be influenced by other materials laid down 

 regionally in the egg and not displaced by centrifug- 

 ing. So far as kno^^Tl, such materials would have 

 no permanent effect upon inheritance unless they 

 were of a plastid character and capable of inde- 

 pendent multiplication. It is customary, therefore, 

 to consider all daughter cells as inheriting the entire 

 genetic complex of the mother cell, and differ- 

 entiation is ascribed to other influences than to 

 sorting out of hereditary materiids. It is referred 

 to the environment in the widest sense, including 

 the relationshij:) (physical and chemical) to neigh- 

 boring cells, and to the external surroundings of 

 the whole embryo. 



