THE MITOTIC CYCLE 



the basophil granules of the cytoplasm, the 'chromidia', had dis- 

 appeared. These bodies must therefore be related in composition to 

 nuclein. A similar treatment of sections of the intervertebral ganglia of 

 the cat resulted in the dissolution of the Nissl granules. The demon- 

 stration that nucleic acid is found in the cytoplasm is thus nearly forty 

 years old. 



In VAN Herwerden's experiments, the changes within the oocyte 

 nucleus, either in the chromatin or the nucleolus were very much less 

 marked. He returned to this subject in a further study (van Herwer- 

 DEN^^) of the effects on the sperm head of both nuclease and i per cent 

 hydrochloric acid. He found that in both fishes and echinoderms the 

 basophily of the sperm nucleus was decreased after hydrolysis by the 

 latter reagent in the cold, but that the mammalian sperm was resistant 

 to this treatment. The same result was obtained in each case by digestion 

 with nuclease. Van Herwerden saw in his results an illustration of the 

 observations of Kossel^* and Kossel and Edlbacher^^ that the 

 linkage between the basic protein and nucleic acid was broken with 

 particular ease in the sperm nucleoprotein of fishes and echinoderms. 

 He further observed that the unripe mammalian sperm head was 

 attacked by the agents which he employed. 



We now know that it is the difference between the nucleic acids of 

 the sperm head and of the egg cytoplasm which is primarily responsible 

 for VAN Herwerden's results. At the time, the conclusion was drawn 

 from these results that the 'nuclease test' could not be used as a reliable 

 index of the presence of chromatin. The verdict of Wilson^^ on this 

 matter, still to be found in the third edition and its reprints is that 'it 

 must be admitted, therefore, that we have no certain means of identify- 

 ing "chromatin" in the cell apart from its morphological history'. 



CH — CH 



II i 



CH GCHO 



\ / 



O 



(i) Furfural 



CH 3— CO— CH J— CH J • COOH 



(ii) laevulinic acid 



HO— CHa— COCH,— CHa-CHO 



(iii) o). hydroxy laevulinic aldehyde 



Figure i — Structural formulae 



The recognition of two classes of nucleic acids preceded this cyto- 

 chemical work of van Herwerden by many years, but until long after 

 this period it was believed that one was found in plants and the other in 

 animals. In 1893 Kossel^' had shown that although nucleic acids from 

 both yeast and animal sources contained the same purine bases, the 



