PROTOZOA 125 



the protoplasm of a cell appears to be alveolar or foam-like in structure. 

 This is because the protoplasm is emulsoidal in character being com- 

 posed of a mixture of many more or less non-miscible substances, 

 some of which are fluid in character, others more of the nature of 

 solids. In such a mixture, the more viscid materials form tiny 

 globules, and each of these is surrounded by a layer of softer material 

 (Fig. 91). Consequently, cytoplasm is alveolar in structure; it has an 

 appearance similar to that produced by the myriads of bubbles in a 

 mass of foam. 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 but the outline of the alveoli, 

 which are not in contact with a firm structure, is more nearly 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 un- 

 yielding surface, their outline becomes rectangular. 



Recent studies in colloidal chemistry and in the microscopic dissection 

 of cells have furnished valuable contributions to the knowledge of the 

 chemical and physical properties of protoplasm. The view has been 

 advanced that protoplasm consists largely of material in a state known 

 in colloidal chemistry as a gel, some portions being firm and viscid 

 and others very soft in character. Procedures which convert such 

 material into a sol or fluid state are said to cause the protoplasm to 

 quickly disintegrate. Certain portions of the cell such as the limiting 

 membrane, the nuclear membrane and the nucleolus are of firmer 

 consistence than other portions, and some cells contain globules and 

 granules of various types. 



The protoplasm of a protozoon may be divided into two main 

 portions: the cytoplasm and the nucleus (Chapter I). 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 some- 

 times 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 a structure 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 which may vary 



