70 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



one viscid and the other more labile. In such a mixture, the viscid fluid 

 forms tiny droplets, and each of them is surrounded by a layer of the less 

 coherent fluid (Fig. 44). The arrangement of the alveoli of the foam-like 

 cytoplasm of a living cell is the same as the arrangement of the bubbles 

 in a mass of foam which is artificially produced. The walls of the outer 

 layer of alveoli, or of alveoli which surround a resistant structure within 

 the cell, are perpendicular to the surface against which they lie. The 

 outline of the alveoli, which are not in contact with a firm structure, is 

 circular; an exactly similar arrangement of the alveoli may be seen in a 

 mass of soapsuds contained in a bottle; wherever the bubbles touch an 

 unyielding surface, their outline becomes rectangular. 



The protoplasm of a protozoon may be divided into two main divisions: 

 the cytoplasm and the nucleus. The cytoplasm, as a whole, may be 

 divided, more or less easily, into a clearer, denser, more resistant outer 

 layer the ectoplasm; and a more fluid, granular, internal portion the 

 endoplasm. Denser, more resistant fibers sometimes run through the 

 cytoplasm and, like a skeleton, serve to fix the shape of the organism in 

 which they exist. 



The nucleus, in its simplest form, is simply an area which is differ- 

 entiated from the remainder of the cell by being more refractile and 

 by being colored more deeply in specimens which have been stained 

 by dyes. It stains deeply because it contains a substance called chro- 

 matin. The chromatin usually occurs in granules; the granules may 

 vary considerably hi size and they are supported upon a linin frame- 

 work. This framework does not stain by ordinary methods and it is 

 probably continuous with, and of the same nature as the substance which 

 forms the alveoli of the cytoplasm. The interstices of the nucleus are 

 filled with nuclear sap. A limiting nuclear membrane may be present, 

 but it is not an essential part of the nucleus. The nuclear material may 

 be all gathered together in a single mass, or it may be distributed in 

 small granules so that, at the first glance, no nucleus seems to be present. 

 Such a nucleus is called a distributed nucleus. 



FUNCTIONS OF THE PROTOZOA. 



Most animals are composed of a great number of cells; a protozoon 

 consists of a single cell. In an animal which is composed of many cells, 

 the various functions of the body are each carried out by a special type 



