482 DANIEL MAZIA 



(4) In animal cells at least, and conceivably in all cells dividing by 

 mitosis, the poles are not an abstraction, but are represented physically by 

 particles called " centrioles ". The polarization of mitosis by these remark- 

 able particles depends on their power of self-reproduction and on the fact 

 that they move apart, following their reproduction, in a definite way that 

 is superficially describable as a "repulsion". The movement is almost 

 certainly not an actual repulsion, and does not follow an inverse-square 

 relationship [14]. While the reproduction and movement of the centrioles 

 is a major problem in the analysis of mitosis, I shall say no more about it 

 here, but will refer to a recent publication of ours [23]. There is a lot to 

 say about centrioles, but not as chemistry. 



In the above summary, I have not discussed the progress that has been 

 made in the electron-microscope study of the mitotic apparatus. This has 

 been reassuring (cf. [24]) to the extent that it has confirmed the "existence " 

 of the reasonable structure that had been inferred from accumulated 

 cytological knowledge, but does not necessarily make life simpler for the 

 chemist with his instinctive homogenizer. 



5. Isolation of the mitotic apparatus : the stability problem 



The mitotic apparatus is a large body, clearly seen in living cells of 

 many kinds. Sometimes it would seem to occupy a very large fraction of 

 the cell's volume. By using eggs of marine animals, which may be obtained 

 in mass quantities and which divide synchronously following fertilization 

 in the laboratory, we may obtain sufficient material for the isolation of the 

 mitotic apparatus for chemical study. The difficulty of achieving such an 

 isolation arises from the fact that the apparatus is so unstable ; if we break 

 open the cell in any of the media that are so satisfactory for other sub- 

 cellular structures, the mitotic apparatus simply falls apart. 



It now seems to me that the instability of the mitotic apparatus is 

 perhaps the most interesting of all the problems of its structure, and 

 perhaps holds the key to many other problems of cell structure which we 

 have been compelled to ignore. I shall return to this point in a later section. 

 Experience other than attempts to isolate the mitotic apparatus attests to 

 its instability. It seems to disappear from the living dividing cell, sometimes 

 reversibly, under many chemical treatments, under high pressures, at 

 extremes of temperature, etc. It loses its characteristic orientation very 

 easily in vivo. Its structure and orientation may well be the expression of 

 an equilibrium between dissociated, and oriented-associated molecules, an 

 equilibrium that is sensitive to many variables. Such an equilibrium has 

 been discussed by Inoue [21]. If this is a proper approach to the stability 

 of the mitotic apparatus — and I now think it is — we could hope to isolate 

 it in one of two ways. The easiest, and the one which first succeeded in the 



