CHAP, iv.] from 1838 to 1851. 331 



referred to a common principle. Both observers had en- 

 deavoured to conjecture the course of events from certain data, 

 supplying by inference what they had not directly observed. 



Nageli about the same time took up a different position as 

 an opponent of Schleiden's theory. In an exhaustive treatise 

 on the cell-nucleus, cell-formation, and cell-growth in plants, 

 the first part of which appeared in 1844 in the periodical 

 founded by himself and Schleiden, he collected together all 

 that had hitherto been observed by himself and others from 

 various points of view. All sections of the vegetable kingdom 

 were once more systematically examined with reference to the 

 occurrence of the cell-nucleus and the different kinds of cell- 

 formation ; all cases of the latter were carefully compared 

 together in their resemblances and differences, in order to 

 deduce from the observed phenomena that which was essential 

 and universal. The first result was, that Schleiden found 

 himself obliged, in the second edition of his ' Grundziige ' in 

 1845, to accept the cell-division established by Nageli in Algae 

 and the mother-cells of pollen as a second kind of cell-forma- 

 tion ; thus began the movement in retreat which was destined 

 to end in the following year with the overthrow of Schleiden's 

 theory. This was effected by the continuation of Nageli's 

 treatise in the third volume of the periodical for 1846. In the 

 first part of his work Nageli had set out by assuming the 

 correctness of Schleiden's assertions, though he was even then 

 compelled to modify them considerably. In the second part, 

 however, in consequence of further observations Schleiden's 

 theory was declared in plain terms to be utterly incorrect, and 

 was refuted point by point. But Nageli was not obliged to 

 confine himself to this negative result ; his comprehensive 

 investigations supplied material at the same time for construct- 

 ing a new theory of cell-formation, which not only took in all the 

 various cases, but declared the principle which lay at the root 

 of all. If we compare this second part of Nageli's treatise with 

 von Mohl's publications from 1833 to 1846, we shall see that 



