324 M. H. V. Mohl on the Structure of Chlorophyll. 



around the cavity of vacuoles in the mucilage, a dense, membra- 

 naceous coat, — this would certainly be an evidence in favour of a 

 membrane having been formed around soft or fluid contents, for 

 a substance which forms a coat over another must be of differ- 

 ent nature from the latter — must be distinguishable from it. But 

 the microscope shows no trace of this distinction ; it shows 

 nothing more than that the structures in question are sharply 

 defined against the cell-sap at their surface. Whether the sur- 

 face of the protoplasm and of the granulose structures proceed- 

 ing from it, are firmer than their internal substance, can never 

 be directly made out by the microscope, just as little as we can 

 see in a drop of water, whether the view of those physicists is 

 correct, who believe that the outer surface of every fluid has a 

 firmer consistence than its internal portions. That in regard to 

 the protoplasm this definite limitation on the outside has very 

 frequently been wrongly taken as evidence of the existence of a 

 membrane, is well known ; I need only refer, in respect to this, 

 to the opinion of Schultz, who looked upon the protoplasm- 

 currents as currents of milk-sap flowing in the ramifications of 

 a vascular system. We see nothing more on the firmer struc- 

 tures of the cell-contents composed of proteine- substances than 

 we do on these currents, which indicate so clearly, by their 

 movement and their continual changes, that their surface is not 

 formed by a membrane ; for all exhibit merely a simple outline. 

 At the same time it is not denied, and I have never denied this 

 (see my observations on the nucleus in the " Vegetable Cell ''), 

 that the surface of these structures, for instance of the nucleus, 

 may and frequently does possess a firmer consistence than the 

 internal portion. But this is not enough to constitute a mem- 

 brane, for it is indispensable to the notion of the latter, that it 

 forms a layer definitely bounded on both surfaces, either, as in 

 the superimposed layers of cell-membrane, agreeing in structure 

 with the adjacent tissue and only mechanically separate from it, 

 or consisting of a special, different tissue ; but it by no means 

 sufiices for the formation of a membrane, that a homogeneous 

 substance possesses a sharply defined surface of firmer consist- 

 ence, if this firmer layer passes insensibly into the rest of the 

 substance, so that no one can determine where the outer layer 

 ceases and the internal substance begins. In such a case, look- 

 ing at the outer surface, we may say it is membranaceously con- 

 solidated ; but we only open the door to confusion, if we use for 

 the naming of this condition the same expression as that which 

 we apply to a peculiar or special layer forming a definite con- 

 trast to the subjacent substance : in ordinary life such a con- 

 fusion may be passed over, but in scientific works, treating of 



