2 CYTOLOGY CHAP. 



organization can be made out in it in the living state (its constituents 

 having nearly the same refractive indices), while when " fixed " by one 

 of the various kiUing and hardening agents, and stained, the structure 

 which is thus made visible varies considerably with the reagents used for 

 fixation and the subsequent staining. It is not proposed therefore to do 

 more than mention the most important views, especially as at present we 

 have no means of correlating structure with function in the case of the 

 cytoplasm. In our present state of knowledge, to decide between these 

 various views is comparatively unimportant as regards general biological 

 problems, apart from biophysics, with which we are not here concerned ; 

 but as we shall see later, the matter stands other\vise with the nucleus, 

 where exact determination of structure and function is often of critical 

 importance for theories of heredity and other problems. 



Views as to the structure of the cytoplasm can be arranged as foUows : 

 (a) the reticular ; (b) the fibrillar ; (c) the granular ; (d) the alveolar. 



(a) The reticular theory. According to this, the cytoplasm consists 

 of a more solid constituent forming a reticulum or network, like a sponge, 

 containing in its meshes a more fluid substance known as the ceU sap or 

 enchylema. In addition, a greater or smaller number of minute granules 

 or microsomes are embedded in the reticulum. 



(6) The fibrillar theory. According to this view the reticulum is 

 not continuous, but is composed of disconnected threads embedded in a 

 matrix (Flemming,^ 1882), 



(c) The granular theory. As developed by Altmann (1893) this 

 depends more upon theory and less upon observation than do the other 

 views. The cytoplasm is supposed to consist essentially of granules, 

 only the largest of which are visible through the microscope. The 

 microsomes mentioned above are examples of these. Each granule is 

 itself a Uving organism or hioblast and bears much the same relation to 

 the cell as the cell itself to the whole organism. As regards the inter- 

 granular substance, Altmann supposed that this is mainly composed of 

 granules below the limit of visibility. Any substance which may be 

 left over between these ultimate granules is non-living matrix. This 

 theory is mainly of historical interest. 



{d) The alveolar theory. The alveolar theory of Biitschli (1892) 

 supposes the cytoplasm to possess a frothy structure similar to that 

 of the emulsion formed when two immiscible fluids are shaken together. 

 The cytoplasm therefore consists of minute drops of one fluid sus- 

 pended in a second, denser fluid, which, being generally small in bulk 

 compared with the included droplets, forms thin films surrounding them 

 like the films of soapy water surrounding the air in a foam of soap bubbles. 



^ References to the exact source of all authorities quoted will be found at the end 

 of the book. 



