THE MITOTIC CYCLE 



acids are concentrated in the isotropic segments of insect striped muscle, 

 though a still greater part of the phosphate there present is due to 

 phosphagen, and is thus unrelated to nucleotides. However, in a 

 previous paper (Engstrom^^^) this author showed that a range of other 

 tissues, the ash after micro-incineration is concentrated in regions where, 

 ultraviolet absorption was high. 



It is probable that in general both divalent cations and phosphate 

 contribute to the ash of the nuclei and chromosomes. Williamson and 

 GuLiGK^^* have analysed mammalian nuclei prepared by the anhy- 

 drous method of Behrens and find therein 2*5 per cent phosphorus, 

 I '35 per cent calcium, and o*o8 per cent magnesium.* These authors 

 calculated the percentage of nucleic acids to which this phosphorus 

 content would correspond, and concluded that at the pH of the living 

 nucleus, the nucleic acids were combined with the maximum possible 

 amount of calcium. If this is so, any further concentration of calcium 

 in the nucleus during mitosis would necessitate the synthesis of more 

 nucleic acids. 



The elasticity of both lamp-brush chromosomes (Duryee^'*) and 

 those of the salivary glands of Chironomus (D'Angelo^"") is markedly 

 reduced by calcium ions. If, therefore, such effects of this ion are shared 

 by all chromosomes in general, and if their elastic properties have any 

 relevance to the changes in form which they undergo during mitosis, 

 a concentration of calcium round the chromosomes might play a part 

 in their contraction at metaphase. 



Further progress in this field demands the use of a method by which 

 both calcium and phosphorus can be recognized with certainty in the 

 same material on a cytochemical scale. Development of the X-ray 

 absorption histospectroscopy of Engstrom^^i at present oflfers the best 

 hope of such progress. 



THE CENTRAL BODY 



As early as the mid-nineteenth century, several authors, among which 

 Derbes^''^ may be mentioned, described the astral rays in various eggs 

 well before their significance as part of the division mechanism of the 

 cell was realized. A quarter of a century later, when the general form 

 of the achromatic figure in the animal cell with its asters and spindle was 

 understood, attention was directed to the nature of the focal point at 

 each pole. Van Beneden^"^ was among the first to describe this 

 'corpuscule polaire' which he had observed in the cells of organisms no 

 less obscure than the Mesozoan Dicyemid parasites of Cephalopods. 

 Later came the concept of the central body as a permanent organ of 

 the cell which remains when the rest of the achromatic figure has 



* JuNGNER^*^" has recently found appreciable quantities of magnesium in DNA and 

 nucleohistone isolated from the calf thymus, and in RNA prepared from yeast. 



106 



