242 PROTOPLASM 



layer, so that we could explain the origin of such stratified 

 cuticles by the consecutive formation of several successively 

 hardened alveolar layers. As has been pointed out, I 

 hence consider it very probable that, as Carnoy and From- 

 mann assume, cell membranes and cuticles might arise by 

 hardening of the outermost layer of protoplasm, i.e. of the 

 marginal alveolar layer. The suggestions of both these 

 investigators as to the actual method in which the process 

 takes place, namely, hardening and chemical modification of 

 the framework, filling up of the meshes with cellulose or 

 solid nitrogenous substance, seems to me at the present 

 moment too hypothetical and uncertain for us to enter more 

 closely into the subject. 



On the other hand, I cannot in any way agree off-hand 

 to regard all membranous envelopes of protoplasmic bodies 

 in general in the same way. I am obliged rather to allow 

 that membranes of this kind can also arise by actual secre- 

 tion, i.e. by means of a substance which emerges at the 

 surface of the protoplasm and hardens to an envelope. 

 Thus I have already, at an earlier period, discussed 

 accurately the reasons which make such an origin very 

 probable for the cyst envelopes of the Ciliata (Protozoa, p. 

 1659). I will only refer here to the fact that certain 

 Ciliata rotate actively by means of their clothing of cilia 

 during the formation of this envelope, which, in my opinion, 

 makes it quite impossible to imagine that this envelope 

 arises by direct modification of an external layer of proto- 

 plasm. The same thing also appears very probable for the 

 shells of the Ciliata, and possibly also those of numerous 

 Flagellata. We may also observe many stages of transition 

 between gelatinous coats and firm enveloping membranes, 

 which, in like manner, point to the same interpretation ; 

 for the gelatinous coats are most certainly actual excretions, 

 as far as we have an accurate knowledge of their origin. 

 Moreover, we have now set foot in a region that really 

 scarcely admitted of being investigated with profit before 

 the nature of the protoplasm itself was accurately discovered. 

 I hope that the further development of the view of the 

 structure of protoplasm which has been set forth by me. 



