CHARACTERISTICS OP CELLS. 257 



tures — as the Amoeba — in which we are quite unable 

 to distinguish either muscles or any other distinct organs. 

 Hence there is always some danger of believing that to be 

 simple which in reality only seems to be so, and which 

 the minuteness or transparency of organization may only 

 conceal from our limited power of vision. 



Plants and animals, if seen at the earliest stage of 

 existence, present themselves to our eyes as an aggregation 

 of transparent cells. Everything prior to the appearance 

 of the cell may, in the actual state of our microscopical 

 knowledge, be considered as not fully and certainly demon- 

 strated ; and therefore it is incumbent upon us to take our 

 starting-point from the simple cell, which is the same, in re- 

 spect to its principal characters, in animals and vegetables. 

 The external coating of a cell is nearly or quite solid and 

 transparent, and with no indication of structure ; while 

 in its interior is found a liquid or solid substance, with a 

 nucleus either adhering to its wall or within its cavity. 

 A nucleolus can sometimes be demonstrated within the 

 nucleus ; and (a state common to all living cells) an in- 

 cessant mutual interchange of materials is going on between 

 the fluid contents and matter external to the cell, by 

 a process termed osmose, or diffusion, which causes a per- 

 petual variation in its relative condition. Chemical reagents 

 give a manifestly different result in the animal and vege- 

 table cell, hence we may conclude that there is an important 

 difference in their chemical composition. The vegetable 

 cell has an extremely fine delicate membrane lining the 

 inner wall, to perceive which we must have recourse 

 to reagents, and then we find the apparently simple cell- 

 wall made up of two layers, each differing in composition 

 and properties. The inner layer has received the name of 

 primordial utricle, and fta composition has been shown 

 to be albuminous ; agreeing in this respect with the form- 

 ative substance of animal tissues. The external layer is 

 regarded as the cell- wall, although it takes no part, essen- 

 tially, in the formation of the cell ; it is composed of cellu- 

 lin, a material allied to the cellulose of vegetable tissues. 

 The contents are more or less coloured : the internal 

 colouring substance is termed endochrome ; when green it 

 is called chlorophyll. 



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