General Discussion 251 



for these cells to die to permit invasion of their lacunae by osteogenic 

 elements and consequent growth in length of the bone. A failure of 

 cell senescence here results in a failure of growth. Just such a failure 

 can be produced experimentally by vitamin A deficiency. In a 

 deficient animal the cartilage cells mature but fail to die. In hyper- 

 vitaminosis A, on the other hand, senescence and death of cells in 

 the cartilage columns is accelerated and this results in a premature 

 obliteration of the epiphyseal plate and a secondary arrest of growth. 

 So, in bone there is this curious situation, wherein cell senescence 

 and cell death are an essential process contributing to growth of 

 the organism. 



Dempsey: You are using 'growth' here in a slightly different con- 

 text because growth, in your example, means the accumulation of 

 extracellular products. 



Huggett : I would like to ask Prof. Wislocki a point arising out of 

 his remark earlier this afternoon that you can get nerve cells re- 

 placed at a molecular level — what other forms of evidence, growth 

 or development in the cells, takes place in addition to the mitotic 

 development? In other words, can you get an amitosis; how much 

 does it exist ; how much does it come into the picture ? 



Wislocki: There are frequent references in the literature to ami- 

 tosis, or direct cell division, in which chromosomes do not form. 

 There has been much discussion as to whether normal cells ever 

 divide by this process (cf. Wilson, J. W., and Leduc, E. H. (1948). 

 Amer. J. Anat, 82, 353). 



Huggett: I raised this point because Arthur Hughes and I have 

 been looking at cuttings of skin and liver in foetal whales. Now, 

 though the whale foetus grows a thousand times faster than the 

 primate foetus in utero, we cannot find any mitotic effect. Now it 

 may be we have not sampled early enough; but for the moment, 

 it looks as if the simplest thing about this fast-growing organism is 

 an almost complete absence of mitosis. 



Montagna: Lack of mitotic figures in fixed material is no index 

 that there is no mitosis. One might ask, exactly when did the tissues 

 die? When were the tissues fixed after the embryo had been re- 

 moved ? It is possible that all the cells which had begun mitosis had 

 completed mitosis before the tissue dies — and is it not a fact that 

 this often takes place in tissue? 



Mitosis is just one of many factors in growth. In wound healing, 

 for instance, mitosis is not abundant even in material fixed quickly, 

 immediately after removal. In growing hair follicles the actual pro- 

 liferative zone, or matrix, is very small. As the cells move up from 

 the matrix they increase many hundreds times in volume ; this is a 

 very important factor in growth. Hollander found that after he 



