132 THE POPULAR SCIENCE MONTHLY 



THE EOLE OF MEMBEANES IN CELL-PEO CESSES 



Br Professor RALPH S. LILLIE 



UNIVERSITY OF PENNSYLVANIA 



THE importance of membranes in vital processes has long been rec- 

 ognized. From the earliest times anatomists have been impressed 

 with the frequency with which thin sheets of solid material occur as 

 elements of structure in organisms. Even elementary methods of 

 analysis show that the materials composing the most various organs 

 often tend to dispose themselves in thin, continuous layers. Thus the 

 entire body is enclosed in an extremely resistant and impermeable 

 layer, the skin. Each of the internal organs has its own characteristic 

 enclosing membrane; the peritoneum lines the body-cavity and invests 

 the intestine and its associated glands, the heart is enclosed in the peri- 

 cardium, the lungs in the pleura, the central nervous system in the pia 

 mater; the muscles are closely surrounded by thin connective tissue 

 sheaths, or perimysia ; the walls of the blood-vessels and of the intestine 

 and other hollow viscera consist of several distinct concentric layers. 

 Various products of animals, like the eggs of birds and reptiles, often 

 show this tendency. Plants also deposit a great part of their structural 

 materials in layers; the wood forms concentric circles; leaves and 

 fruits have thin and often waterproof membranous coverings; the 

 orange is partitioned by a system of membranes and each smaller por- 

 tion of pulp has a membrane of its own. The instances, in fact, are in- ' 

 numerable. Evidently the tendency to deposit material in thin con- 

 tinuous sheaths is highly characteristic of organisms. 



This much was clear at a time when anatomists were limited to 

 direct and unaided vision. When the microscope came into use the ex- 

 istence of a similar tendency soon became evident in the minutest tissue- 

 elements. The living substance exhibited itself everywhere as mi- 

 nutely subdivided by innumerable thin partitions, or membranes, giv- 

 ing it a characteristic honeycomb-like or cellular structure. These par- 

 titions isolate the enclosed portions of living substance and render 

 them at least mechanically separable. Hence the conception that each 

 of these minute membrane-enclosed masses of gelatinous or viscid " pro- 

 toplasmic " material is an independently living entity, or elementary 

 physiological unit, gained ground, and, as all know, has been univer- 

 sally adopted in biology. The name " cell," originally applied to the 

 minute spaces themselves, has been transferred to the protoplasmic 

 mass within, by whose activity the enclosing membrane is itself formed. 



Thus it was early recognized that cells tend to separate materials 



