CELL MEMBRANES AND CUTICLES 241 



ginal alveolar layer in general should be classed as a cell 

 membrane, since, neither in the physical nor the histological 

 sense, is it originally such, yet I consider it probable that 

 it may frequently develop, as the result of solidification, 

 into a firm membrane, which can be termed a cell 

 membrane. 1 



The grounds for this assumption are, in the first place, 

 that there is no doubt that at least the outer limiting 

 lamella of the alveolar layer, which in Protozoa I termed 

 the pellicle, has often, as a matter of fact, become of a firm 

 consistence. We must always admit this where definite 

 forms of cells occur, which are other than spherical, since 

 they are not possible without the surface being of a firm 

 nature. That the whole alveolar layer may become firm is 

 a legitimate conclusion, from the circumstance that in 

 certain Ciliata and in Nassula a second radiate layer occurs 

 beneath it, which may sufficiently prove the firmness of 

 the outer one. The radiate arrangement of the trichocyst 

 layer (cortical protoplasm) in Urocentrum and Paramcecium 

 also depends in part upon similar reasons. 



Further there are well-formed resistant chitinous shells, 

 which have been isolated from the protoplasmic body, and 

 which possess the characteristic structure of the alveolar 

 layer. This has been longest and best known for the shell 

 of Arcella ; similar but more flexible shells are also found 

 in Cochliopodium and probably some other Ehizopods. 2 I 

 consider it very probable that these honeycombed envelopes 

 are derived directly from an alveolar layer. It was further 

 shown above that the cuticle also of Phascolosoma and 

 Brancliiobdella consists of several or even numerous layers, 

 of which each one has somewhat the structure of an alveolar 



1 Thin flat cells in which the whole body consists for a considerable 

 distance of only a single layer of alveoli (see above, the cells of the blood 

 capillaries, and the connective tissue cells of the ischiadic nerve), prove con- 

 clusively that not all cells can possess a cell membrane in Carney's sense. In 

 such cells this simple layer of alveoli plays in itself the part of a marginal 

 alveolar layer on both surfaces, and this fact proves most definitely, in my 

 opinion, that the ordinary marginal alveolar layers also belong to the proto- 

 plasm, although they may assume the character of firm cell membranes. 



2 In the same way also in Flagellata, such as Trachelomonas. 



16 



