PHYSIOLOGICAL 403 



exhibit de-differentiation. In certain cases the less specialised ele- 

 ments in a culture become cannibals as regards de-differentiated 

 elements, that is to say, the relapsed descendants of cells that were 

 originally specialised. 



The tissue-culture method practically owes its origin to Prof. 

 Ross Harrison, who made pioneer experiments with isolated nerve- 

 cells in 1910; it has been ably developed by Carrel and others, and 

 it has great possibiUties. It makes it possible to distinguish the 

 potentialities that are resident in the tissue itself in a normal culture 

 from the reactions that follow the introduction of drugs or extracts 

 of other tissues, or hormones, or extracts of malignant tumours. It 

 is a quaint technique, but it may enable investigators to detect or 

 delimit the factors that control, restrain, inhibit, or stimulate the 

 growth of tissues. It is one of the methods by which descrip- 

 tive embryography is gradually being advanced into causal 

 embryology. 



How Long Can a Cell Live? — A "Big Tree" or Sequoia may 

 live for over three thousand years, but much of the tree, namely, 

 the hard wood and outer bark, is merely dead skeleton. From the 

 continuance of the life of the organism as a whole one cannot argue 

 to the longevity of particular cells, except in cases where no replace- 

 ment occurs and the individual cells remain alive. Thus it is generally 

 believed that when the brain of a backboned animal has once 

 reached its normal size, there is no further multiplication of its 

 nerve-cells; therefore, a living nerve-cell of that brain has lived 

 since the animal's maturity at least. It is generally believed that 

 there is no increase in the number of our brain-cells after birth. If 

 this is quite correct, then those nerve-cells of a centenarian's brain 

 that remain alive must have lived for a hundred years. The American 

 botanist, D. T. MacDougal, has recently pointed out that the cells of 

 the pith of the tree cactus (Carnegiea giganiea) may continue to grow 

 and function for more than a century. It does not seem to be the 

 living matter or protoplasm that grows old in a cell; it is rather 

 what may be called the furniture of the cell-laboratory, that is to 

 say, the more stable plasmic framework within which metabolism 

 occurs. Thus, if a cell is not very highly differentiated, it should 

 live longer than one that is specialised. As for single-celled organisms, 

 which divide periodically, it seems that many of them must have 

 evaded natural death altogether. 



The Physical Basis of Life. — Huxley used these words, "the 

 physical basis of life", as a sort of definition of protoplasm or 

 genuinely living matter. While there is "one kind of flesh of men, 

 another flesh of beasts, another of fishes, and another of birds", 

 Huxley was laying emphasis on the fact that all kinds of "flesh" have 

 a similar physical basis. Speaking of the structural units or cells that 

 build up the body of plant or animal, von Mohl said in 1846: "The 



