THE LIVING SUBSTANCE. 25 



their fleeting existence. But none the less are they areal differ- 

 entiations of structure ; and none the less are they associated with 

 physiological fimctioji of the substance. 



In Metazoan eggs, as of starfish, echinus, rotifer, fish, or 

 frog, in Metazoan larvae even, the same holds equally true. 



It may be said for living protoplasm, that of making pellicles 

 there is no end. Yet the pellicles when made differ one from 

 the other as markedly as, in many cases, do nuclei, or cell 

 membranes, or skeletons, or any other coarser, complex structure 

 of the substance. 



The optical network of Biitschli's structure is found in all 

 protoplasmic masses I have seen, to be locally emphasized so as 

 to form optically greater separation between areas of the sub- 

 stance by marked partition lines, plates, or membrane-like thick- 

 enings of the lamellar film. 



Such modifications are to be grouped, both in structure and 

 origin, with the pellicles surrounding external masses and 

 forming contact surfaces. 



Like these, they are actual thickenings of interalveolar stuff 

 along lines of the physical lamellae, and mean that here the 

 mass of interalveolar foam has been augmented, or has under- 

 gone physical modification of some sort. 



VVe must now begin to distinguish verbally between the 

 necessary physical, or purely physical, pellicle and such thicken- 

 ings as these. For the former I shall say physical pellicle, or 

 lamellar substance; for the latter merely pellicle, or pellicular 

 plate, thickening, or membrane. 



[21] Wherever a finer foam structure can, directly or indi- 

 rectly, be traced in a pellicle, it follows, from the nature of the 

 case, that the outer surface of that pellicle is in its turn covered 

 by a pellicle. Hence, wherever pellicles are augmented by 

 interalveolar material they are double ; and actual contact with 

 environment, whether this be internal, or external, to the mass, 

 is made by the pellicle of the finer froth. 



Fine instances of this are seen in the pellicular formations of 

 Vorticellidae. 



Because of phenomena found associated with flux of pellicu- 

 lar substance, it becomes impossible to set a limit to such 



