264 PROTOPLASM 



podia and cortical layer of Rhizopods is produced from 

 alveolar protoplasm, and is capable in like manner of re- 

 conversion into it. The investigation of the pseudopodia 

 of Gromia Dujardini furnished a decisive proof of this fact. 

 Moreover the fibrous alveolar structure, which is distinctly 

 exhibited by these apparently homogeneous pseudopodia here 

 and there after suitable fixation and coloration, is a sure 

 proof that they do not lack the honeycombed structure, but 

 only that as the result of special conditions it is no longer to 

 be observed in life, nor even to a great extent after fixation. 

 Hence arises the question whether it is possible to supply 

 a more or less plausible explanation for the disappearance 

 of the structure. At an earlier date (1890) I had already 

 suggested, as a possible explanation of this phenomenon, 

 that the alveoli became widened, whilst there was a con- 

 sequent thinning out of their walls to such a degree of 

 fineness that they ceased to be visible. I also pointed 

 out upon physical grounds that a foam, which as a matter 

 of fact manifests a great similarity to solid bodies in 

 its appearance, would do so the more the thinner its walls 

 became. I did not see until later that this suggestion also 

 obtains a certain amount of support from the observations of 

 Mensbrugghe (1882) upon the tension of thin fluid lamella?. 

 Mensbrugghe has shown that the tension of these lamellae 

 increases when their thickness sinks below a certain limit, 

 and that the lamellae then behave as firm elastic membranes, 

 whose resistance to further stretching constantly increases 

 (see on this point Lehmann, Mblecularphysik, Bd. i. p. 25*7). 



This would agree with the fact that the homogeneous 

 protoplasm which occurs in Ehizopods seems to possess a 

 greater viscidity or firmness than the more fluid and 

 distinctly reticular internal protoplasm. It is usually 

 sought to explain this fact by ascribing a somewhat greater 

 density to the homogeneous ectoplasm a notion which, 

 as a matter of fact, obtains no support from its optical 

 appearance. 



From Plateau's calculations on soap-bubbles, it follows 

 that the thickness of the lamellae of macroscopic soap-foams 

 may be very small. From the interference colours which 



