THE STRUCTURE OF PROTOPLASM 



237 



Fig. 115. — Alveolar protoplasm, 

 sketched from a living Euplotes. 



tion. Viewed through a microscope, protoplasm usually presents 

 the picture of a fine dispersion of globules and particles of differ- 

 ent sizes, freely suspended in a liquid medium (Figs. 114, 3). 

 Many of the particles are mere 

 specks ; others are relatively large 

 (maximum, 10 ^) and are dis- 

 tinguishable as droplets. When 

 the specks predominate, the 

 protoplasm may, though possibly 

 inexactly, be characterized as 

 a suspension of granules. If, 

 however, the specks prove to be globules, as are the larger 

 particles, then the structure is that of an emulsion. These 



possible configurations of the visible 

 parts of living matter are the bases 

 of three theories of protoplasmic 

 structure, viz., granular, emulsion, 

 and alveolar. The propounder of 

 the last mentioned theory introduced 

 the word alveolus (a small alveole, 

 or cavity) to indicate symmetrically 

 arranged globules which resemble 

 small vacuoles and assume, because 

 of pressure, an angular form (Fig. 

 115). Neither alveolus nor vacuole 

 is a well-chosen term, as there are no 

 cavities or empty sacs in protoplasm. 

 Modifications of the alveolar or 

 emulsion structure of protoplasm 

 occur, though the differences are 

 mostly superficial. When alveoli 

 are not under pressure, and therefore 

 less symmetrical (Fig. 3), they 

 are referred to as alveolar spheres. 

 When the "cavities" are of the 

 nature of small sacs, the structure is 

 termed vacuolar (Fig. 116). It is, 

 however, impossible to differentiate clearly among a vacuole, 

 a sac, a globule, an alveolus or an alveolar sphere. All of 

 these structures are but modifications of one and the same 



Fig. 116. — Vacuolate pro- 

 toplasm of the egg cell of 

 Ceratozamia. {After C. J. 

 Chamberlain.) 



