164 UTR1CULAR STRUCTURES. 



cavities are larger and exist in smaller numbers, and when 

 the mucilage generally is more diluted and transparent. 

 The cavities have, on the contrary, more the aspect of 

 real, mere cavities, when they are small and numerous, 

 and when the mucilage is dense and opaque. 



These cavities contain water. They are seen in most 

 cells which are passing from the condition in which the 

 cavity is wholly filled with homogeneous mucilage, to 

 that in which the mucilage is deposited upon the wall as 

 a mucilaginous layer. They also occur, not unfrequently, 

 at a subsequent period, when the cells contain a homoge- 

 neous mucilage besides the solid matters ; I have seen 

 them in this late stage of the life of the cell, from the 

 cells of Conferva, Siphonece, and Hyphomycetes, upwards 

 to the parenchyma-cells of the Phanerogamia. They are 

 here a normal phenomenon of cell-life. 



The following is the probable mode of origin of these 

 cavities. Larger or smaller quantities of water separate 

 from the mucilage, and from physical laws acquire a 

 globular form. From the loss of water, the mucilage 

 contracts and becomes more dense. If the drops of water 

 lie long unaltered in the mucilage, the layer of the latter 

 in contact with the water coagulates through its influence, 

 as is always more or less the case with mucilage or albu- 

 men in water. In this way originate in the cell-contents 

 the colourless pseudo-utricles possessing a special mem- 

 brane. In conformity with their origin we never see any 

 other contents but colourless fluid in them, and never 

 observe any alteration in their apparent membrane. 



That this explanation of the normally occurring utri- 

 cular cavities is correct, is proved by the abnormal forma- 

 tion of similar cavities, either when mucilage or albumen 

 is mixed with water, or when we allow water to act endos- 

 motically upon a cell containing homogeneous mucilage. 

 In both cases, similar globular, colourless, sharply-defined 

 cavities are frequently produced, the mucilage at the same 

 time contracting and becoming more dense and opaque. 



These cavities in the mucilage must by no means be 



