32 INTRODUCTION TO CYTOLOGY 



it may readily be observed (see Chambers, 1924). Seifriz (1924a) states 

 that the viscosity of protoplasm is on the average about like that of 

 glycerine and seldom below that of machine oil. According to Heilbrunn 

 (1926c), the viscosity of the granule-free cytoplasm of the Arbacia egg 

 is 0.02 and that of the Cumingia egg 0.04 (water = 0.01). With the 

 granules included the values are 2 or 3 times as high as this. Myxomy- 

 cete protoplasm is found to be 15 or 20 times as viscous as water (Heil- 

 bronn, 1922). The internal protoplasm of Paramoecium is said to be 

 8,000 times as viscous as water (Fetter, 1926), although this may be due 

 in part to fibrillar structures present. The effects of a variety of agencies 

 on viscosity have been ascertained.^^ The viscosity changes occurring 

 during meiosis, syngamy, and cell-division will be referred to in a later 

 chapter. 



As already stated, cytoplasm nearly always contains visible globules 

 and other particles. These represent vacuolar material, chondriosomes, 

 and more or less transitory nutritive substances of various types. Some 

 cells show no such elements or only chondriosomes, but this condition is 

 rather exceptional. The smallest of the other visible granules, the 

 "microsomes," are almost universally present, according to Chambers 

 (1917, 1924). In the echinoderm egg there are, in addition, many larger 

 "macrosomes" (called "alveolar spheres" by Wilson, 1899), which 

 measure 3 or 4^ in diameter and seem to represent nutritive matter. 

 Other bodies, such as fat and oil globules, are also present. All of these 

 may be so abundant that the ground substance, or hyaloplasm, is almost 

 invisible. Likewise, in plants especially, minute vacuoles may be so 

 numerous that the protoplasm has an alveolar or foamy appearance. 

 Fibrillar differentiations may further complicate the picture. It will be 

 readily recognized that the optical appearance of a given sample of 

 protoplasm depends very largely upon the kinds of inclusions present, 

 their size and arrangement, and the degree to which they are crowded 

 together. 



A fact which is both striking and very significant is that cytoplasm 

 may be deprived of all the above inclusions without losing the power of 

 carrying on certain of its characteristic activities. Hyaline pseudopodia 

 amputated from granular amoebae are irritable and move in a typical 

 amoeboid manner. Centrifuged sea-urchin eggs can be cut into two por- 

 tions, one with all the visible granules (except oil globules) and the other 

 with none, after which both portions may be inseminated and undergo 

 cleavage (see Chambers, 1924). This means that the visible inclusions, 

 although they should be regarded as a part of the living system to the 

 degree in which they are active in protoplasmic reactions (see p. 46), 

 are not an indispensable part of that system; and that "it is in the appar- 



J9 Heilbrunn (1920c, 1924, 1925/, 1929a&), Heilbnmn and Young (1930), Weber 

 (1922, 1923, 1927), Scarth (1924), and others. 



