INTRODUCTION 



41 



times filled with clear watery fluid (contractile vacuoles) which is 

 emptied to the outside at regular intervals, or sometimes filled with 

 fluids which are not discharged (stationary vacuoles, or cavulse of 

 Wetzell). The same form, when fixed with a good killing agent, 

 and properly stained, gives a permanent picture of the granides, 

 vacuoles, and other cell parts as they were at the instant of fixation. 

 The nucleus now stands out as the most conspicuous part of the 

 cell, while the granules are seen to be of different sizes and to react 

 differently after treatment with dift'erent stains. 



Fig. 20. — Phalansterium digllatum St. Individuals (/) in branched gelatinous colony. 



(After Stein.) 



In most cases the finer physical structure of the protoplasm can be 

 seen both in the living cell and after fixation. It is best described 

 as a foam structure similar to the bubbles of soap suds but with 

 "bubbles" or alveoli of microscopic size. Imagining an optical 

 section through a soap suds in which granules of finely-powdered 

 carmine have been distributed by stirring, the picture presented 

 would be a network or meshwork of water, soap, and carmine, and 



