THE STRUCTURE OF CELLS 



theory has exerted considerable influence upon contemporary thought, 

 and may be given in brief outline. The whole protoplasm, including 

 the nucleus, is conceived of as in a physical condition resembling 

 an emulsion, the more fluid mass filling the cavities (which are 

 very small, 1-2 p in diameter), whilst the walls are composed of a 

 more viscous substance. Such an emulsion obeys certain well-known 

 physical laws, and the relation of the alveolar walls both to each 

 other and to the free outer surface can be theoretically defined. 

 Solid heterogeneous particles enclosed in the emulsion, if too large 

 to lie in the substance of the walls, are surrounded by a surface 

 film in which the alveoli are arranged, as they also are on the free 

 surface, somewhat like the cells of columnar epithelium. Move- 

 ments may occur in the whole mass, as the result of disturbances 

 in the surface tension of the superficial alveoli, and movements pro- 

 duced in emulsions in this way closely resemble the streaming and 

 other movements of protoplasm. The reticular 

 and filar appearances are also to be attributed 

 to disturbances, due to causes, in the interrelation 

 of the alveoli, or they may represent the optical 

 section of the alveolar walls themselves. 



Now it is quite possible to convince oneself 

 that such an alveolar structure does actually exist 

 in many cases, although, as Biitschli himself 

 admit?, it is not always to be so identified. It 

 seems probable, however, that on the whole it 

 does represent truly the appearance of protoplasm 

 under certain (and commonly occurring) condi- 

 tions, but it also seems equally clear that these 

 conditions are not necessarily always fulfilled. 

 For it is essential for the production of such an 

 appearance that there shall be at least two non- 

 miscible fluids of different refractive index, and 

 if either of these conditions is not realised, or is 

 temporarily in abeyance, it will follow that the 

 FI<>. 3. alveolar appearance must also be absent or dis- 



The foam structure appear. And we are acquainted with so many 



of the protoplasm of a- ,, ii i .. r 



grearine. important series of changes in the relations of 



the various protoplasmic constituents to each 

 other that it is hardly necessary to postulate the permanence of 

 those conditions of which an alveolar structure is the consequence. 

 Thus it would seem that an easy modus vivendi might be reached 

 which would render it possible, whilst recognising the heterogeneity 

 of the substances included under the generic term Protoplasm, to 

 admit that at one time an alveolar, at another a filar, or a reticular 

 appearance might occur. A fibrillar structure is certainly present 

 during nuclear division, and although the extreme adherents of the 



