ITS CHEMICO-PHYSICAL AND MORPHOLOGICAL PROPERTIES 35 



are more frequently found ; these either distend the cell or render 

 it somewhat solid. When there has been a considerable develop- 

 ment of such substances, the protoplasm may again assume a 

 frothy appearance, as in Actinosphcerium (Fig. 15), or it may 

 become transformed into a network structure, as in a Tradescantia 

 cell (Fig. 13), the only difference being that the interstices are 

 filled with substances denser than sap. 



Na cv 



cv 



FIG. 15. ActinospTicerium Eichliorni (after R. Hertwig, Zoologie, Fig. 117) : M medullary 

 substance, with nuclei (n) R peripheral substance, with contractile vacuoles (ev); Na 

 nutrient substances. 



The most perfect examples are often seen in animal egg-cells 

 The exceedingly large size, which is attained by many of these, is 

 not so much caused by an increase of protoplasm, as by the storing 

 up of reserve materials, which vary very much as to their chemical 

 composition, being sometimes formed and sometimes unformed 

 substances, and which are intended for future use in the economy 

 of the cell. Very often the egg-cell appears to be almost entirely 

 composed of such substances. The protoplasm only fills up the 

 small spaces between them, like the mortar between the stones of 



