350 THE FORMS OF CELLS [ch. 



and external*, and the way in which particles move freely hither 

 and thither within, all manifestly tend to the production of swelhng, 

 rounded surfaces, and to their great predominance over plane sur- 

 faces in the contours of the organism. These rounded contours 

 will tend to be preserved for a while, in the case of naked protoplasm 

 by its viscosity, and in presence of a cell-wall by its very lack of 

 fluidity. In a general way, the presence of curved boundary 

 surfaces will be especially obvious in the unicellular organisms, and 

 generally in the external form of all organisms, and wherever 

 mutual pressure between adjacent cells, or other adjacent parts, 

 has not come into play to flatten the rounded surfaces into planes. 



The swelling of any object, organic or inorganic, living or dead, is bound to 

 be influenced by any lack of structural symmetry or homogeneity f. We 

 may take it that all elongated structures, such as hairs, fibres of silk or cotton, 

 fibrillae of tendon and connective tissue, have by virtue of their elongation 

 an invisible as well as a visible polarity. Moreover, the ultimate fibrils are 

 apt to be invested by a protein different from the "collagen" within, and 

 liable to swell more or to swell less. In ordinary tendons there is a "reticular 

 sheath," which swells less, and is apt to burst under pressure from within; 

 it breaks into short lengths, and when the strain is relieved these roll back, 

 and form the familiar annuli. Another instance is the tendency to swell of 

 the "macro- molecules" of many polymerised organic bodies, proteins among 

 them. 



But the rounded contours which are assumed and exhibited by 

 a piece of hard glue when we throw it into water and see it expand 

 as it sucks the water up, are not near so regular nor so beautiful as 

 are those which appear when we blow a bubble, or form a drop, or 

 even pour water into an elastic bag. For these curving contours 

 depend upon the properties of the bag itself, of the film or membrane 

 which contains the mobile gas, or which contains or bounds the 

 mobile liquid mass. And hereby, in the case of the fluid or semifluid 

 mass, we are introduced to the subject of surface-tension: of which 

 indeed we have spoken in the preceding chapter, but which we must 

 now examine with greater care. 



* The protoplasm of a sea-urchin's egg has a viscosity only about four times, 

 and that of various plants not more than ten to twenty times, that of water itself. 

 See, for a general discussion, L. V. Heilbrunn, Colloid Symposium Monograph^ 1928. 



t D. Jordan Lloyd and R. H. Marriott, The swelling of structural proteins, 

 Proc. U.S. (B), No. 810, pp. 439-44.3, 1935. 



