12 THICKENING PROCESS. [BOOK i. 



such as are wanting in young cells. That is certainly pos- 

 sible; but it is also possible, that the fact as shown by anatomy 

 is altogether otherwise. With the same kind of reason as 

 that which leads to the inference that a new layer is formed 

 externally, we may assume that in a cell originally consisting 

 of true cellulose, that substance subsequently, and without 

 any alteration of its relative position, is absorbed and 

 replaced by an essentially different chemical compound; 

 or that cellulose remains and a new compound is depo- 

 sited between its molecules, interfering with the behaviour 

 of cellulose towards iodine and sulphuric acid. Such an 

 infiltration might occur without any visible thickening of the 

 layer, either if it were not in very great abundance, or if the 

 growth of the membrane in a lateral direction, in consequence 

 of the expansion of the cell, were to make room for the 

 deposit of a considerable quantity of some foreign compound. 

 In these cases, the possibility of which no one will call in 

 question, a layer would indeed be formed altogether new in 

 a chemical aspect, but no alteration in anatomical relations 

 would appear : and from such a chemical transformation no 

 conclusion should be drawn as to the order in which the dif- 

 ferent layers of the cell-membrane originate ; because such 

 metamorphoses may take place as readily in the last as in the 

 first formed layer. If we admit the possibility of such a 

 metamorphosis, it must also be conceded that the chemical 

 reaction of any particular layer affords no sure means of 

 recognising it as a peculiar anatomical layer ; for it may be 

 easily imagined, that in different cells, the layers corresponding 

 to each other in an anatomical point of view, may exhibit a 

 great distinction in regard to their chemical transformations. 

 Until well-grounded experience has taught us which of the 

 cases, that have here been mentioned as possible, really 

 occurs in nature, we can only allow ourselves to be guided, 

 in the recognition of the different layers and the deter- 

 mination of the order in which they make their appear- 

 ance, by their anatomical relations ; and although in very 

 many cases the influence of chemical re-agents affords an 

 excellent means by which we are enabled to distinguish 

 the individual layers of cell-membrane, which without this 



