236 PROTOPLASM 



of the purely fibrillar structure of protoplasm, derives a continuous 

 limiting membrane to the protoplasm in the same way as has been 

 already described for the membrane of the vacuole, namely, by a 

 cementing together of the fibrils by means of a special cement 

 substance. 



For the rest, the majority of investigators who have occupied 

 themselves with protoplasmic structures do not, as a rule, touch 

 more closely upon the question of the formation of the outer 

 surface of the protoplasm. 



From what lias been brought forward it follows that 

 the representatives of the supporting framework theory 

 require special assumptions to explain the limitation of 

 the framework towards the surrounding medium ; for the 

 facts of the matter are not as Leydig represented them, but 

 every observation tends to show that a continuous non- 

 porous limiting margin always covers the surface of the 

 protoplasm. Now since protoplasmic bodies can often be 

 burst by pressure or cut in pieces at will, and then still 

 show again a sharp continuous border at their surface 

 — an experiment which has in fact been done so often, both 

 on the animal and vegetable side, that it seems unnecessary 

 to bring forward special examples here — the adherents of 

 the supporting framework theory are forced to assume either 

 that mysterious forces immediately close the exposed meshes 

 again and again, or that this closure is effected by a 

 formation of cementing substance, etc. 



Such assumptions are, as has been said, entirely un- 

 necessary for the alveolar theory. According to it the 

 constant presence of a pellicle-like continuous border is a 

 direct consequence of the supposed structure, and every 

 drop of protoplasm that is separated off finds itself in 

 exactly the same conditions again as the original proto- 

 plasmic body, and will therefore in like manner possess a 

 limiting border. 



id) Alveolar Layer 



We now come to a further relation of the highest 

 importance with regard to the external limiting layer of the 

 protoplasm, which in my opinion is sufficient to decide the 



