20 AN INTRODUCTION TO ZOOLOGY 



structure of protoplasm. Of these the three following may be 

 mentioned. 



(1) The Reticular Theory maintains that protoplasm consists 

 of a living network of anastomosing fibers (Fig. 4, B) ; between 

 these a variety of non-living substances, such as water and fat, 

 may be present. 



(2) The Alveolar Theory (Biitschli, 1892) supposes that pro- 

 toplasm has a foam-like structure somewhat similar to a mass of 

 minute droplets such as exist in an emulsion of oil and water 



(Fig. 4, A). The originator 

 of this theory was able to 

 make artificial emulsions 

 which showed a striking 

 resemblance to living sub- 

 stance. He maintained that 

 the fibrous network, de- 

 scribed bv the adherents of 



A "R 



FIG. 4. Diagrams illustrating (A) the alve- the reticular theory, repre- 

 olar, and (B) the reticular theory of sented the walls of the alvC- 

 protoplasmic structure. (From Dahl- olar bodies which had been 

 gren and Kepner.) cu t acrO ss. 



(3) The Granular Theory (Altman, 1892) asserts that proto- 

 plasm is composed of innumerable living granules arranged either 

 along fibers or among alveoli; nothing is essential, however, ex- 

 cept the granules. 



None of these theories has universal approval, but the first two 

 have been more widely accepted than the last. Many recent 

 authorities believe that protoplasm is highly variable in structure. 

 A fourth view held by many observers is " that the various types 

 described above are connected by intermediate gradations and 

 may be transformed one into another " (19, p. 27). 



Protoplasm is a mixture of extremely complex chemical com- 

 pounds; these are, at least in part, extremely labile and therefore 

 are apt to vary at different times. It is impossible to study 

 living substance, since even the most careful methods of analysis 



