CELLS IN DIVISION 



disadvantages. There are no adequate methods for testing the chemical 

 composition of the ash, and furthermore it is probable that artefacts 

 result from the movement of inorganic salts within the cell during 

 chemical fixation. 



The first observations on cells in division which were made by means 

 of micro-incineration methods are due to Scott/^^ ^^^ who described 

 how the whole ash of the cell during mitosis seems to be concentrated 

 in the chromosomes both in mouse spermatogonia and in epithelial 

 cells of the frog skin. In the interphase nucleus, nuclear membrane 

 nucleoli and residual chromatin all leave their recognizable traces of 

 ash. In Scott's preparations, it appeared that inorganic material was 

 gradually concentrated in the developing chromosomes during pro- 

 phase. A similar concentration of cellular ash in nuclei and chromo- 

 somes was observed by Funaoka and Ogata^^' in ova and cells of the 

 oviduct ofAscaris, and in pollen mother cells of the bean. Barigozzi^^® 

 micro-incinerated salivary gland chromosomes and found that their 

 Feulgen-positive bands have a high content of ash. However, a warning 

 against generalizing too readily from these results is to be found in the 

 work of LucAS^^^ on the ash of Opalinids, for he found no residue 

 whatever in the chromosomes of these Protozoa although they too are 

 Feulgen-positive. 



It was assumed by Scott that the ash of the nucleus and the chromo- 

 somes consisted mainly of calcium and magnesium. Two Polish workers 

 (Kruszynski;1^° ^^^ Baginski^^^) endeavoured to apply chemical tests to 

 ashed preparations under the microscope by adding suitable reagents 

 with micropipettes held in a Chambers micromanipulator. Thus the 

 presence of calcium in the ash was shown by the formation of char- 

 acteristic crystals of gypsum when sulphuric acid and alcohol were 

 added. Scott and Packer^^^ confirmed the presence of calcium and 

 magnesium in the ash of the cell by a technique which combines micro- 

 incineration with electron microscopy. A tissue section is mounted on 

 the cathode of a special instrument, and is there ashed in situ. When 

 heated further, the ash of the material emits electrons which are imaged 

 on a fluorescent screen. It is claimed by these authors that the emission 

 from the ash is mainly due to atoms of calcium and magnesium. In 

 electron micrographs of smooth muscle-fibres made in this way, 

 ScoTT^^* has shown that the nuclei are prominent. 



Other evidence, however, indicates that the ash of nuclei and chromo- 

 somes is not entirely composed of salts of calcium and magnesium. 

 User and Goodspeed^^^ consider that the ash of chromosomes in 

 pollen mother cells is mainly due to phosphates from the nucleic acids, 

 the phosphorus content of which is high; for instance Gulland^^^ 

 states that there is 9*3 per cent phosphorus in dried DNA from calf 

 thymus. Engstrom^^' has shown that both the ash and the adenylic 



H 105 



