APPENDICES 1449 



division remain obscure, but what takes place has been well 

 observed; and this knowledge has formed, as already said, one of 

 the foundation-stones on which a great biological superstructure has 

 been reared. In 1883 it led van Beneden to another foundation- 

 concept, that in the fertilisation of the egg-cell by the sperm-cell, 

 there is in each of the combining elements the same number of 

 chromosomes, and that the number is half that which is charac- 

 teristic of the somatic cells of the species. In the maturation both 

 of the egg-cell and the sperm-cell there is a unique kind of nuclear 

 division — called "reducing" or "meiotic" — by which the number of 

 the chromosomes is reduced to a half. Thus when fertilisation is 

 effected, the normal number is restored, half being contributed by 

 either parent. This discovery was followed by the demonstration, 

 first due to Riickert, Zoja, and Haecker, that the paternal and 

 maternal chromosomes and the initiatives they carry are divided 

 with precise accuracy among the daughter-cells into which the 

 fertilised ovum divides. There is cumulative evidence in support of 

 the modem "chromosome theory of inheritance", which maintains 

 that man^^ if not all, of the hereditary factors or genes are carried 

 in linear order, like beads on a string, in the colloidal chromosomes. 

 Part of the evidence is afforded by the remarkable way in which 

 the behaviour of the chromosomes in maturation and fertilisation 

 harmonises with the facts of Mendelian inheritance. 



A momentous event, though its biological import was slow of 

 becoming clear to many, was Thomas Graham's study (1861) of 

 osmosis, and its disclosure of the properties of matter in a colloidal 

 state. The biological relevance of this became luminous when it was 

 realised that living matter is in a colloidal state, with countless 

 ultra-microscopic particles and droplets suspended or dispersed in 

 a more or less liquid medium, and affording in proportion to the 

 total mass an enormous surface on which a multitude of chemical 

 and physical actions may take place. 



It was an important step, with far-reaching consequences, when, 

 about 1899, W. B. Hardy showed that active protoplasm has no 

 visible structure under the highest power of the microscope, but 

 presents to the ultra-microscope the characteristics of a colloidal 

 system. Careful observers had described the reticular, fibrillar, or 

 other intimate structure of dead and fixed protoplasm, but the 

 complex microscopically visible framework was shown by Hardy 

 and others to be an artefact, a result brought about by the methods 

 of fixation. Active protoplasm is a fluid system, with not less than 

 70 per cent, of water, and contains innumerable mobile particles and 

 suspended droplets. From this "sol" state the active protoplasm 

 may sink temporarily or fatally into a "gel" state, like set jelly or 

 boiled white of egg. 



There are often microscopically visible particles in a cell or in a 



