SECT. 8.] CELL-MEMBRANES. 13 



size of the cells descends on the one hand, as in many cells, the 

 blood-cells, etc., to 0002 — 0*003 of aline, and attains on the other, 

 as in the cysts of the semen and the ganglionic globules, the size of 

 o - 02 — 004 of a line. The largest animal-cells are certain gland- 

 cells of insects, which, according to H. Meckel, measure up to o - i 

 of a line; the yelk-cells or ova, especially of birds and amphibia, 

 and some animals consisting of a single cell which, like certain 

 gregaringe, attain 07 of a line. 



The membrane of cells is mostly very delicate, smooth, scarcely 

 capable of being isolated, and bounded by simple outlines; more 

 rarely of considerable firmness and measurable thickness. In 

 some places where it is thicker it distinctly presents pores ; and it is 

 very possible that such pores are a very widely distributed phe- 

 nomenon. In the interior of cells, there are constantly found, at a 

 certain period, one or several nuclei, besides fluid and granules in 

 varying proportions and of different natures. Cells which only con- 

 tain fluid are rare (fat-cells, blood-cells, cells of the chorda) , and it is 

 colourless or reddish ; the majority contain, besides, other corpuscles 

 (elementary granules, elementary vesicles, perhaps also crystals) 

 in greater or less numbers ; young cells, as a rule, contain few 

 granules, while older ones have many, which are very often more 

 densely grouped around the nucleus, or only occupy a single place 

 (coloured ganglionic globules) . 



The chemical composition of cells is still very obscure. The 

 contents possess, in the majority of cells, certain universally dis- 

 tributed materials, which occur dissolved in the nutritive fluid or 

 in the cyto-blastema, such as water, albumen, fat, extractive mat- 

 ters, and salts ; a nitrogenous substance, precipitable by water and 

 diluted acids, is very widely distributed. This substance resembles 

 mucus, and tenders the microscopical examination of tissues very 

 difficult, causing them to appear turbid and granular instead of 

 clear and bright. Many cells contain other compounds, as those 

 of the liver, blood, etc. The cell-membranes consist of a nitro- 

 genous substance, which, in young cells, is undoubtedly a proteine 

 compound, as may be concluded from their solubility in acetic acid 

 (in part even without heat), and in diluted caustic alkalies. Subse- 

 quently, the membrane in many cells becomes insoluble, but is far 

 from being so in all (c. g., in the blood corpuscles, deepest epidermic 

 and epithelial cells, and the cells of glandular follicles), and occa- 

 sionally approaches in texture the substance of the elastic tissue. 



The Cell Nucleus is a spherical or lenticular crystalline, or yellow- 



