THE CELL-MEMBRANE. 345 



of the wall. On this hypothesis the membrane is composed 

 of a number of minute more or less rounded protoplasmic 

 bodies, the dermatosomes, which so long- as the cell-wall is 

 immature and capable of growth are united together by 

 delicate filaments, also composed of living matter. The 

 cellulose is formed by these dermatosomes, which become 

 embedded in it. New dermatosomes are continually 

 constructed from the substance of the threads, so that 

 the wall grows by a process of intercalation somewhat 

 recalling Naegeli's intussusception. Successive layers of 

 the membrane, however, may be formed so that the 

 thickening of the adult wall may be due to apposition, while 

 each layer is formed and increased by intercalation of 

 dermatosomes. 



The membrane which bounds two contiguous cells can 

 more or less easily, as we shall see, be separated into two 

 layers, often by merely mechanical methods, more frequently 

 by chemical ones. Wiesner hence infers that the threads 

 which connect the dermatosomes of two contiguous layers 

 can be more easily ruptured than those which exist in the 

 substance of the several layers of the wall. 



The great value of this hypothesis will appear later. 

 For the present we may note that according to it the whole 

 substance of the membrane is in contact with protoplasm 

 and not merely its internal surface. We have therefore the 

 possibility of an active chemical change of the substance of 

 the wall and not the probability of merely a decomposition of 

 inert compounds, commonly described as a process of de- 

 gradation. 



The generally received idea of the cell- membrane is 

 thus that of a homogeneous sheet of cellulose, formed in 

 some way by a kind of secretion from the protoplasm, and 

 only showing differentiation of its substance when it under- 

 goes such a change as that of lignification or one of the 

 others spoken of. In a tissue which has much-thickened 

 walls, it is not difficult to recognise a middle layer lying be- 

 tween the cells in the substance of the wall, and to notice that 

 the tissue can be disintegrated into its constituent cells by 

 the process of solution of this layer. This uiiddlc /aiuei/a, 



