UNITIES OF LIFE 



brane of which enclosed a fluid content, and a solid 

 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 arose by a sort 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 

 membrane, which was not only regarded 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 

 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 Schultze showed that the 

 external membrane 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 not 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 sarcode in 1835. Max 

 Schultze further showed that this "sarcode" was iden- 

 tical with the "cell mucus" of the plant cells which 

 Hugo Mohl had designated "protoplasm" in 1846, and 



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