UNITIES OF LIFE 



brane of which enclosed a fluid content, and a soHd 

 smaller body inside this, which R. Brown had recognized 

 as the nucleus in 1833. They compared the cell, as a 

 microscopic individual, to an organic crystal, and 

 thought it aro^e by aTsort of crystallization in an organic 

 medium (cytoblastema) ; in this the central nucleus 

 would serve as starting-point like the nucleus of the 

 crystal. 



In the first twenty years (1839-59) of the cell theory 

 it was a fixed principle that there were three essential 

 parts, of the cell. Firstly, there was the strong outer (ii 

 membrane, which was not only regard^ed as a protective 

 covering, but also credited with a great deal of impor- 

 tance as an element in the building of the organism. In . 

 the second place, there was the fluid or semi-fluid con- 

 tent (the sap); and, thirdly, the firm nucleus enclosed A^ 

 in the sap. In order to give a clearer idea of the rela- 

 tive thickness and disposition of these parts, the cell 

 was compared to a cherry or a plum. The soft flesh of 

 this fruit (corresponding to the cell sap) can, with diffi- 

 culty, be separated from the external firm skin or from 

 the hard stone within. A great step in advance was 

 made in i860, when Max Sch\iJtze showed that the 

 external mernbrane was an unessential and secondarily 

 formed part of the cell. It is, as a fact, altogether 

 wanting in many, especially young, cells of the animal 

 body. They are naked cells without any membrane. The 

 distinguished anatomist also proved that the co-called 

 "cell sap" — the real body of the cell — is n^t a simple 

 fluid, but a viscous, albuminous substance, the inde- 

 pendent movements of which had long been known in 

 the rhizopods, and which the first to study it carefully, 

 Felix Dujardin, had described as sarcodc in 1835. ^^^^^ 

 Schultze further showed that this '*sarcode" was iden- 

 tical with the "cell mucus" of the ])lant cells which 

 Hugo Mohl had designated "proto^sm" in 1846, and 



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