THE PRESENT POSITION OF CELL-THEORY. 109 



already been set forth by other authors in their criticisms of 

 similar theories, particularly by Biitschli {loc. cit., p. 195) 

 and O. Hertwig, both of whom occupy themselves with 

 Altmann's views, which are to all intents and purposes 

 identical. Only a few of the most important points need be 

 touched upon here. 



It is certainly a remarkable fact, and confirmed by 

 abundant experience, that many of the constituent parts of 

 cells are produced by divisions which recall the divisions of 

 the cell itself. The nucleus is the most important and the 

 most familiar constituent of the cell : it is within the experi- 

 ence of every biologist that nuclei are never observed to 

 originate neogenetically, but always by division of a pre- 

 existing nucleus. The chromatin elements of the nucleus may 

 be shown to be composed of minute particles, the so-called 

 chromosomes, and these reproduce themselves by division, 

 and are never observed to originate neogenetically. The 

 same statement holds good for the centrosomes, for 

 chlorophyll corpuscles and for various kinds of chroma- 

 tophores. It is not to be denied that these facts, which 

 become more and more familiar to the working microscopist, 

 appear to lend a powerful support to the theory of 

 biophors ; in a limited sense they may be said to be a proof 

 of the statement that the cell is an organised body. 

 Whether, as Wiesner claims to be the case, there are many 

 other constituents of cells which similarly reproduce their 

 kind by division, and are never observed to originate 

 independently, may for the present be left out of considera- 

 tion. The evidence that amylum grains and granules of 

 various kinds behave like the centrosomes in this respect, is 

 as yet too slight, and the observations are too conflicting to 

 enable us to come to a judgment without entering into a 

 mass of detail which is not wholly relevant to the question 

 at issue. But there is at least one criticism which is worthy 

 of mention, namely, that of Biitschli, who points out {loc. cit., 

 p. 200) that among the strongly staining granules of proto- 

 plasm there are bodies which are not actually constituents 

 of the protoplasm but are symbiotic micro-organisms. 

 The existence of such organisms, which have been called 



