378 CELL HEREDITY 



iiUMital stiiiitili siR'li as tlic absence ot liiilit. AltliDiigli plastids arc a 

 special ci'll structure, others l)eha\i' .siniilarlw The kinetosomes at the 

 bases of Hagella seem to have autonomy in reproduction and the chance 

 of their loss ina\' he increased b\ the presence ol druy;s such as acridinc 

 dves. Some forms, such as Panntwrhiin hursaria. C'hlorlujdia viri.ssiuia, 

 and Convoluta roscojjcth'iis (a Hatworm), have endosvmhionts, green al- 

 gae which h\e in the cytoplasm to the mutual benefit of both organisms. 

 Thev are passed from cell to cell at disision and, in the last two forms, 

 to the egg at ovulation. The svmbionts behave as cell particulates and 

 mav be lost upon cultivation in certain enxironments. These and many 

 other cases provide models for the possible behaxior of similar entities 

 known to occur in the somatic cells of higher organisms. 



INFECTIOUS HEREDITY 



Recombination mechanisms that do not involve .sex pro\idt> another 

 means whereby somatic cells may vary. Although claims are beginning 

 to be made, no sure evidence has been presented for transformation or 

 for fusion and exchange between cells in tissue culture. Nor has clear 

 evidence for genetic exchange come from a study of chimaeras in plants 

 or animals where the soma consists of cells from different individuals. 

 In plant graft-hybrids, cells of different genotype may exist side by side, 

 as some layers of cells come from the scion and some from the stock. 

 Similarly, when animal tissues are grafted, there is opportunity for gen- 

 etic exchange between cells. Claims have been made that this does oc- 

 cur. When a pigmented patch of skin is transplanted into a white area 

 of the same mammal, pigmentation spreads beyond the margins of the 

 graft. These newly pigmented areas can be propagated. The cytoplas- 

 mic extensions of the pigment-forming cells in the skin can be ob,ser\ed 

 to fuse with others, and are thought to permit the spread of pigment- 

 forming particles from one cell to another. The possibility that the 

 spread of pigmentation is due to the migration of the pigment-forming 

 cells themseKes has not been rigorously excluded, howexer. 



Viruses proxide a \ehicle for the transfer of genetic material from cell 

 to cell. The study of animal viruses has been facilitated by the use of 

 tissue-culture techniques; when spread on sheets of fibroblast cells, the 

 viruses form plaques which can be counted and classified morphologi- 

 cally. From these placjues pure lines may be established of many 

 viruses, such as those causing poliomyelitis, influenza, and encephalitis. 

 Their growth within single cells in monodispersed cultures can also be 

 examined, and in all regards they are similar to bacteriophage, except 



