292 PROPERTIES CONFERRED BY COLLOIDAL CONSTITUENTS 



Now the lipoid constituents of the cell may be assumed to be gener- 

 ally impenetrable by substances which are insoluble in fats and oils, 

 so that these must seek entry to the cell through the interstitial spaces 

 between the lipoid constituents of the superficial membrane of the cell. 

 If, therefore, these intersitital spaces filled with a solution or gel of 

 protein, were so constructed as to be narrower at one end than at the 

 other, the superficial membrane of the cell would evidently be more 

 readily permeable in one direction than in the opposite. 



The most usual spatial arrangement of the various structures or 

 constituents of a cell is that of Radial Symmetry. The primitive 

 arrangement of strictly radial symmetry so frequently displayed in 

 spherical cells becomes modified or distorted in those cells, such as the 

 majority of epithelial cells, which, through mutual compression or for 

 other reasons, have assumed a columnar, stratified or flattened outline. 

 In such cases the radial arrangement of structures may be confined to 

 the sides or margins of the cell, and differ in character in the protoplasm 

 underlying the various facets of the cell. 



FIG. 15 FIG. 16 



FIGS. 15 and 16. These figures illustrate the effect of curvature of the surface of a 

 radially dispersed emulsion in producing funnel-shaped interstitial pores between the 

 radiating columns of lipoid globules. In Fig. 15 both surfaces of the cortical layer 

 being plane, the diameter of the interstitial orifices are the same at the exterior and 

 interior surfaces. In Fig. 16 the cortical layer being curved, the interstitial orifices 

 are narrower upon the un der than upon the upper surface of the cortical layer. 



A radial arrangement of the ultramicroscopic fat-granules of the cell 

 would obviously lead to the formation upon the surface and in the 

 subjacent protoplasm of minute Funnel-shaped Pores, of which the 

 interstitial openings would be permeable to substances soluble in water, 

 while the walls, being composed of fat-granules, would be impermeable 

 or with difficulty permeable by such substances. The interstitial 

 openings at the margin distal from the center, from which the fat- 

 granules radiate, would be relatively large, while at points lying nearer 

 to the center of radiation, that is, in general, deeper within the cell, 

 the diameter of the pores would be very considerably contracted. 



Substances which are soluble in water might evidently penetrate 

 such a cell with relative ease, since a relatively large proportion of the 

 exterior cell-surface would consist of the water-soluble phase of the 

 emulsion, but they would issue from the interior of the cell with 

 relative difficulty, since a relatively large proportion of the area which 

 they would have to traverse to find an outlet would consist of the 

 lipoidal phase. If the modifications of radial symmetry which are 

 so characteristic of Epithelial Cells should result in the confinement of 

 this structure to one surface or facet of the cell, it is obviously possible 

 that one-sided permeability of a tissue composed of such cells might 



