402 LIFE : OUTLINES OF GENERAL BIOLOGY 



made to study living cells under the microscope by the "dark- 

 ground" or ultra-violet-light methods — or, simplest of all, by staining 

 the living cells. 



With this last method, striking results have been obtained, special 

 dyes revealing special structures within the cell in an unambiguous 

 manner. To discuss these here is impossible; but there are points 

 in the theory of vital staining which deserve attention. 



In order to stain the interior of the cell, the dye must be able to 

 penetrate the impalpable membrane which forms the boundary of 

 the cell. According to Overton's famous theory, now quarter of a 

 century old, and not yet displaced though regarded as insufficient, 

 this membrane consists of lipoid or fat-like substances, and a dye 

 which dissolves in such substances can readily enter the cell; and 

 this, indeed, is generally found; but some stains quite insoluble 

 in lipoids may be accepted by the cell, especially in certain cases, 

 such as the cells of parts of the kidney, which are specialised for 

 the purpose of removing foreign substances from the blood. Here it 

 is supposed that the uptake of the lipoid-insoluble dyes is an active 

 ph}'siological process (since it disappears if the cells are depressed 

 by narcotics), while the lipoid-soluble dyes enter passively, seeping 

 in by diffusion as they would into a droplet of oil. 



Other theories of the action of vital stains are not lacking; they 

 invoke the internal acidity of the cell, or the magic word adsorption; 

 but, true or false, the "lipoid" theory outlined above is generally 

 found helpful in trying to think clearly in regard to vital staining. 



TISSUE CULTURE.— Our conception of the life of the organism 

 must include the remarkable fact that isolated pieces of tissue may 

 continue living away from the body altogether. A single nerve-cell 

 may live and grow in a suitable culture for a week or more, and a 

 fragment of tissue may be cultivated in viiro for years! Indeed, if 

 care be taken to secure for the isolated fragment appropriate food, 

 a supply of oxygen, a suitable temperature, and aseptic conditions, 

 there seems to be little in the way of limit to its continued vitaUty 

 in a glass tube, or on a glass plate, or in some other such arrange- 

 ment. The life of isolated fragments of tissue from a chick embryo 

 has been prolonged for ten years — a longevity not less than that of 

 the hen into which the embryo might have grown! Throughout all 

 that time there was growth as well as life, and the growth-rate was 

 practically uniform throughout. 



Undifferentiated tissues, whether embryonic or malignant, con- 

 tinue to proliferate in vitro with little histological change or none at 

 ail. They simply give rise to other cells like themselves. More 

 specialised tissues often show extensive cell-division, even when 

 this is not their usual behaviour in the intact adult animal, but 

 ultimately they sink into an indifferent or embryonic type. They 



