THE CELL-MEMBRANE. 351 



in the middle of the present century two opposite views of 

 the composition of the wall or of the interpretation of the 

 facts which have been observed. 



A further controversy has been conducted with regard 

 to the composition of the middle lamella or, as it was once 

 called, the intercellular substance. The constant existence of 

 this part of the membrane was first put forward by von Mohl 



(17) in 1835, and his views were much debated, Mayen 

 opposing and Schleiden supporting them. Mulder and 

 Harting, as mentioned above, distinguished pectose as 

 forming at any rate part of this layer ; Payen considered it 

 formed of mineral pectates, principally those of calcium and 

 potassium. Unger took up the position that its substance, 

 differing from cellulose, was first formed in the tissue and 

 that the cell-wall is a product of its transformation. Wigan 



(18) joins Mayen in denying its existence as a separate 

 layer, and maintains that it is only a product of transforma- 

 tion due to chemical or physical modification of the oldest 

 layers of the cell-membrane. 



Schleiden on the other hand held it to be a true secre- 

 tion of the cells which is caused to accumulate on the out- 

 side of the membrane. This, on the whole, is the view of 

 Schacht, who says that like the cell-wall itself it is probably 

 a product of the primordial utricle. Schacht writes very 

 guardedly on the subject, suggesting that it is not certain 

 that it increases in thickness as the cell-wall Qfrows, but 

 admitting that it seems thicker between adult cells than 

 between young ones. Its reactions according to this author 

 are that it is insoluble in concentrated sulphuric acid, soluble 

 in alkalies, and destroyed by oxidising agents more rapidly 

 than the cell-wall proper. Also, it is more capable than the 

 latter of resisting putrefactive changes. 



Unger's view of this substance being the antecedent of 

 cellulose, is supported by Dippel in 1851 (19). This author 

 describes it as consisting of a kind of jelly which persists as 

 such for some time, cellulose being formed upon its surfaces. 

 As development proceeds it becomes converted into cellu- 

 lose, but even then it only turns brown and not blue on 

 treatment with iodine and sulphuric acid, this difference of 



