36 PRINCIPLES OF ANIMAL BIOLOGY 



marbles are entirely separate from one another, being held apart by thin 

 films of the syrup. It is not possible to pass from one sphere to another 

 without passing through the supporting liquid; but all parts of the sup- 

 porting Hquid are directly continuous. These relations are shown in 

 Fig. 22. Following the usual terminology^, the separate spheres are called 

 alveoli, the supporting liquid the interalveolar substance, and the structure 

 of protoplasm (or of an}^ emulsion) is said to be alveolar. 



Protoplasm has also been hkened to a foam, and the term foam is 

 sometimes applied to it; but, strictly speaking, the alveoli of a foam are 

 composed of a gas, not a liquid. 



The alveolar and interalveolar materials are probably not homogeneous, 

 nor uniform in all parts of the cell. Although the interalveolar material 

 is continuous, and semi-liquid, diffusion must be slow in it, so that in 

 different parts of the cell it is composed of different substances. In like 

 manner, the alveoli probably differ from each other, and there is no 

 assurance that a single alveolus consists of but one substance. Hence, 

 although one sometimes speaks of the alveolar "substance, "and the inter- 

 alveolar "substance," they are not so in the chemical sense. They are 

 not the same thing always, they have no chemical identity. Protoplasm 

 is always a mixture. Of what this mixture consists and something of 

 its chemical nature is stated below. But it is important even at this 

 stage to state explicitly that protoplasm is living matter, not living sub- 

 stance except as the term substance is used to mean matter. 



While the alveolar structure thus far described is the usual one in the 

 protoplasm, there are occasionally found appearances that indicate other 

 types of organization. Sometimes, even with the best microscopes, 

 living protoplasm shows no alveoli, but appears to be entirely homogene- 

 ous. In some examples of apparently homogeneous protoplasm, the ultra- 

 microscope, by means of oblique illumination against a dark background, 

 has demonstrated a host of minute particles. This is taken to mean that 

 the substance under examination is a colloid, that is, consists of multi- 

 tudes of particles of greater than molecular size suspended in a liquid. 

 But a colloid is thus in certain respects not very different from the emul- 

 sion just described, and some of the properties of a colloidal mixture 

 are also those of an emulsion. Furthermore, from the colloidal nature 

 of protoplasm that does not reveal an alveolar structure one may infer 

 that at least the interalveolar substance of other protoplasm is prob- 

 ably colloidal. 



Sometimes, and in some kinds of cells, fibers are also seen in the proto- 

 plasm. In certain instances these fibers are undoubtedly distinct struc- 

 tures immersed in the general emulsion. In other cases, as in dividing 

 cells, they may result from a rearrangement of the alveoli. If the alveoli 

 be placed in rows, the liquid between them will have the appearance of a 

 fiber, an arrangement which is only temporary. Appearances must be 



