334 Theory of Cell-formation [BOOK n. 



shape of cells with advancing growth depend materially on 

 whether they enlarge equally in all parts of their circumference 

 or not. These considerations, obvious as they are, were now 

 for the first time pointed out and fully appreciated. 



The reader who is already familiar with our subject will 

 recognise in the passages adduced from Nageli without further 

 explanation the essential principles of the modern theory of 

 cells, especially if he compares them with the views pro- 

 pounded at the same time and previously by Schleiden, 

 Unger, and von Mohl. But, as might be expected, the further 

 investigations, which were pursued with zeal during the suc- 

 ceeding twenty years and produced a considerable literature, 

 did much to enlarge and perfect Nageli's theory in many 

 of its details and to correct it in some minor points; the 

 theory itself facilitated this process by supplying a scheme 

 to which the investigation of special questions could readily 

 be referred. Whether the nucleus is a solid body or a ve- 

 sicle, whether in the division of a mother-cell into compart- 

 ments the wall of partition always grows from without inwards 

 or is formed simultaneously over its whole surface, whether it 

 is originally composed of two laminae or of one which is 

 afterwards differentiated, these and many other questions 

 were decided in course of time. 



Schleiden's theory was now definitively set aside, a deeper 

 insight was obtained into the nature of the cell, and the ideas 

 connected with the word became broader and more profound. 

 The knowledge of the formation of cells showed that the cell- 

 walls, which had been hitherto regarded as the important part, 

 are only secondary products, that the true living body of 

 the cell is represented by its contents and especially by the 

 protoplasm. Alexander Braun, relying on numerous re- 

 searches into the lower Algae, expressed himself in 1850 

 (' Verjiingung,' p. 244) to the effect that it is an inconvenience 

 that the word cell is used at one time to designate the cell 

 with its wall, at another time the cell without its wall, or again 



