20 



PHYSIOLOGY 



end._.L- 



above theories can be regarded as applicable to all forms of protoplasm, 

 but that during the life of a cell its protoplasm, as observed under the 



microscope, may be either hyaline and 

 structureless or may present any of the 

 structural modifications described above, 

 according to its state of nutrition and the 

 form in which its metabolic products are 

 laid down in the cell. Of course it is 

 possible that, even in the apparently 

 hyaline protoplasm, a structural differen- 

 FIG. 5. Diagram of a cdi. tiation is still present, but is invisible 



highly magnified. (SCHAFER.) ,, ,. ., ... 



owing to the minute size of its constituent 



p, protoplasm, consisting of 

 hyaloplasm and a network of 

 spongioplasm ; ex, exoplasm ; 

 end, endoplasm, with distinct 

 granules and vacuoles; 

 c, double centrosome ; n, nu- 

 cleus; n', nucleolus. 



parts or an identity of refractive index 

 between the alveolar walls and their con- 

 tents. The fact that every chemical differ- 

 entiation occurring within the colloidal mass 

 will tend to cause differences of surface 

 tension, and therefore formation of droplets, shows that an alveolar 

 structure, i.e. one in which there is a large number of surfaces sepa- 

 rating heterogeneous mixtures 

 inside the cells, must be of 

 very common occurrence, even 

 in cases where it is not detect- 

 able under the microscope. 

 Such a structure must be 

 present, at any rate, in those 

 cases where, apart from the 

 existence of a solid cell wall, 

 the cell presents a certain 

 degree of rigidity and resist- 

 ance to deforming stress. 



ULTRAMICROSCOPIC STRUC- 

 TURE OF PROTOPLASM. Since 

 the study of the behaviour of the 

 cell shows that it must possess a 

 much more complex structure or 

 organisation than that which is 

 revealed by the microscope, one, 

 that is to say, which permits of 

 the spatial differentiation of the 

 different chemical processes that 

 may occur at one and the same 

 time in the protoplasm, many 

 theories have been put forward of 



FIG. 6. A, protoplasm of an epidermal cell of 

 the crayfish; B, foam-like appearance 

 of an emulsion of olive oil. (BuTSCHLi.) 



an ultramicroscopic cell structure. Though Spencer in 1864 spoke of physiological 

 unit* out of which protoplasm could be regarded as made up, and Darwin (1868) 



