THE STRUCTURE OF CELLS 



clearly recognised the importance of the formative substance of 

 the cell, to which in 1846 he gave the name it now bears, viz. 

 Protoplasm, the same word that had already been employed six years 

 earlier by Purkinje to designate the formative substance met with 

 in the animal embryo. 



Speculation was already aroused as to the possibility of 

 instituting a comparison between vegetable protoplasm and animal 

 sarcode, and Dutrochet had, as early as 1824, and still more 

 definitely in 1837, indicated the general similarity which underlies 

 the structure of animals and plants. But it was reserved for Cohn 

 to clearly formulate in 1853 the real features of identity between 

 them, and to express the definitely reasoned view that they were, 

 in all essentials, composed of the same kind of substance. Cohn 

 was well fitted for the task, by his acquaintance with the lower 

 organisms of both animal and vegetable kingdoms. Max Schultze 

 in 1861 further elaborated this resemblance, and his convincing 

 demonstration at once gained the assent of all who were competent to 

 form an opinion on the question. Moreover, Schultze clearly saw 

 that it is the protoplasm (in the widest sense of the term) which 

 essentially constitutes a cell, and he, like Leydig, defined it as a 

 mass of protoplasm containing a nucleus. About the same time 

 also Virchow, in his celebrated aphorism, " Omnis cellula e cellula," 

 crystallised the correct view as to the general mode of origin of new 

 cells. 



But although the essential facts of cell-structure and development 

 thus gradually emerged from the earlier and cruder notions, the 

 finer details, and especially the relations of the nucleus, long 

 remained obscure. The origin of this body, and its connec- 

 tion with the rest of the cell-contents, was not understood, and 

 a very general view was held that it disappeared (as indeed is in a 

 certain sense correct) at each cell-division, to be formed afresh in 

 the new daughter cells. It is true that so long ago as 1841 Kemak 

 had put forward the statement that the nucleolus and nucleus gave 

 rise by direct fission to the corresponding structures in the 

 daughter cells, arid indeed that the whole process of cell -division 

 was thus inaugurated ; but his views (which for a few cases 

 are really well founded) appeared to be not generally applicable, and 

 thus it transpired that even in the middle of the century the 

 nucleus came to be commonly regarded as an organ of but secondary 

 importance, and this even by so eminent an investigator as 

 Briicke. It was not until the publication of Strasburger's mag- 

 nificent work on the cell- and nuclear-division in 1875 that the 

 nucleus received its proper share of attention. Strasburger, some 

 four years later, like Virchow, in another connection, before him, 

 defined the modern position in the phrase "Omnis nucleus e nucleo." 

 The researches of the brothers Hertwig, Van Beneden, Flemming, 



