XXX 



cell-wall is formed not immediately round the nucleus (cytoblast), as 

 Sclileiden taught, but round a granular mass in the midst of which 

 the nucleus lies. In this paper also he incidentally corrected the 

 erroneous view that the granules in the contents of the pollen-grain 

 represent spermatozoids, and he observed in certain cases the presence 

 of two nuclei in the grain, though he failed to appreciate the full 

 meaning of the fact. But his comprehension of the nature of the cell 

 was still far below the high level which is marked in his memorable 

 paper " Zellenkerne, Zellenbildung und Zellenwachsthum," which 

 appeared in the 'Zeitschr. fur wiss. Botanik.' In that paper his chief 

 objects are the extension of Robert Brown's discovery of the nucleus 

 (1831) to the principal families of Cryptogams, and the investigation 

 of the processes of cell-multiplication. With regard to the former 

 point, he arrives at the wide generalisation that the nucleus is present 

 in all classes and orders of plants, and that its absence from any 

 plant-cell has not been, and probably cannot be, proved ; and he also 

 recognises that the nucleus is not a solid mass, but a vesicle, with a 

 proper membrane enclosing fluid granular contents with one or more 

 nucleoli ('Zeitschr. fur wiss. Botanik,' Heft 1, 1844, pp. 68 et seq.). With 

 regard to the latter point, he gives up the universality of the Schlei- 

 denian theory for which he had so stoutly contended, and admits (loc. 

 cit., Heft 3 and 4, 1846, p, 49) that, in vegetative organs at least, 

 cell-multiplication is effected by division, " free cell- formation " being 

 restricted to the reproductive organs. 



The chief cause of ISTageli's change of view on the subject of cell- 

 formation was an interesting discovery which, he tells us, he made 

 whilst investigating Algae at Naples in the summer of 1842. He 

 recognised that the " mucus " in the cell forms a continuous lining to 

 the wall of healthy cells, to which he gave the name " Schleiinschicht " 

 (loc. cit., Heft 1, p. 91). It is true that Kiitzing had, in the previous 

 year ('Linnsea,' 1841), made a similar observation, also in the Algae, 

 but he had failed to interpret it correctly : he termed it " Amylidzelle," 

 thinking that it was, or could be changed into, starch. Nageli, on 

 the contrary, ascertained that this layer consists of granular "mucus," 

 which at an earlier stage more or less fills the cell -cavity, and at a 

 later stage forms a parietal layer ; and further, that its reactions show 

 it to be nitrogenous. Having discovered this, he felt that the results 

 of his two earlier papers were no longer convincing, since the 

 phenomena which he had regarded as the expression of processes of 

 free cell-formation could be ascribed, with greater probability, to the 

 contraction of the " Schleimschicht " away from the cell- wall. Sub- 

 sequently (loc. cit., Heft 3 and 4, 1846, p. 52) he carried this discovery 

 to its final stage, asserting that the " Schleimschicht" is never absent 

 "Prom living cells, that it is, in fact, itself living, and that it is from it 

 and by it that the non-nitrogenous cell- wall is formed at its surface. 



