INTRODUCTION 



41 



with low powers of the microscope. In a living form what we 

 actually see under the microscope in most cases is the external zone 

 of protoplasm which, as the surface of contact between the organ- 

 ism and the outer world, has become modified in various ways. 

 Such outer differentiations are usually transparent so that the 

 nature of the internal protoplasm may be made out in more or less 

 detail. This is particularly true of the so-called "naked" forms 

 such as Amoeba proteus, etc., in which the surface protoplasm is 



■■ WMv I \ Y V 



-** ^ 



1 1 



Fig. 22. — Phalansterium digitatum St. [ndividuals (/) in branched gelatinous colony. 



(After Stein.) 



only slightly different from the internal substance and is made up 

 of living material. Here the entire organism is living protoplasm 

 which appears as a drop of fluid substance, grayish-white in color, 

 viscid in physical character but tenuous and with no tendency to 

 mix with the surrounding water. In such living cells, internal 

 movement of the protoplasm is manifested by the streaming (cyclo- 

 sis) of distinct granules, some of which are more refractile than 

 others, but which are present in all cells, and invariably character- 



