CHAP, iv.] from 1838 to 1851. 327 



is impossible to conceive of a case of cell-formation more easy 

 to observe, or less reconcilable with Schleideri's theory. It 

 was mentioned in the first book, that Hedwig and Vaucher 

 were acquainted with the remarkable process of the formation 

 of zygospores in the alga-genus Spirogyra ; but this as late as 

 Schleiden's time was not regarded as an example of cell-forma- 

 tion, and his view was really a step in advance, since it brought 

 a process, so highly peculiar according to existing ideas, under 

 the general conception of cell-formation. 



The systematic elaboration of the theory of cells, founded on 

 careful observation and mature reflection, began with the year 

 1844. Almost at the same time in this year appeared Nageli's 

 detailed enquiries into the occurrence of the cell-nucleus and 

 into cell-division, von Mohl's observations on the primordial 

 utricle and its behaviour in the process of cell-division in young 

 tissue, and lastly those of Unger on merismatic cell-formation 

 (cell-division) as a general mode of proceeding in the growth of 

 organs. As these observers were chiefly concerned to test the 

 correctness and general applicability of Schleiden's theory, they 

 necessarily paid special attention to the general occurrence of 

 the cell-nucleus and to its position on the side of the cell- 

 wall, for these were the points most accessible to observation 

 and criticism. The discussion of these observations disclosed a 

 defect in the current phraseology, in which the word cell was 

 commonly understood to mean only the cell-membrane, but 

 sometimes included everything belonging to and contained in 

 the cell ; hitherto moreover the protoplasm of the cell had not 

 been sufficiently distinguished from the rest of the cell-contents. 



Nageli and von Mohl arrived simultaneously at a clearer 

 understanding of these points; von Mohl recognised the 

 primordial utricle (1844) as a component part of the cell- 

 contents and not belonging to the cell-wall, and explained the 

 part which it plays in cell-division ; in 1846 he arrived at a clear 

 conception of the protoplasm as a peculiar substance distinct 

 from the other contents of the cell and gave it the name it still 



