OF THE CELL THEORY 169 



a fluid or gelatinous substance in which are often 

 present minute particles or granules. 



Furthermore the action of reagents of various 

 kinds shows that the network or reticulum is of 

 different character, presumably of different composi- 

 tion, to the substance filling the meshes, and that in 

 some cases the network itself may vary in different 

 parts. Examples of such reticular protoplasm are 

 frequent among Protozoa as for example in the 

 genera Noctiluca, Trachelius, and many other Infu- 

 soria ; while extreme cases of reticulation or vacuo- 

 lation are seen among the Heliozoa, Radiolaria, and 

 many of the Foraminifera e.g., Globigerina, in which 

 the protoplasm has a characteristic bubbly or frothy 

 appearance from the great number and size of the 

 vacuoles. The contents of the meshes are usually 

 fluid, or semi-fluid, and a study of their behaviour 

 under various conditions, with a comparison of 

 the mode of formation of fat cells, suggests strongly 

 that the reticulum or network is the active part of 

 the protoplasm, and that the substance filling the 

 meshes is of secondary importance to it. Of this 

 view, a most suggestive application has been made 

 with regard to the structure of striated muscular 

 fibre, which appears to consist of a reticulum, the 

 strands of which are arranged in regular geometric 

 pattern, and the meshes filled with a more fluid 

 sarcous substance. It has been suggested that the 

 reticulum is the active agent by contraction of 

 which shortening of the muscle is produced ; and 

 that the reticulum of a striated muscle fibre cell is 

 equivalent to the reticulum of a Protozoan or of any 



