CELLS. 45 



cells, that of 0'02-0'04 of a line. The largest animal cells are the 

 yelk-cells or ova, especially those of birds and amphibia, and a few of 

 those animals which consist of single cells, these, in certain Gregarinse, 

 attaining 0*7 of a line. 



The membrane of the cells is generally very delicate, smooth, hardly 

 separable, and marked by a single contour, rarely of any considerable 

 density or measurable thickness ; with our present optical instruments 

 it exhibits no structure of any kind. In the interior of the cells there 

 are invariably found, at a certain time, one or many nuclei, besides 

 fluid and granules of various proportions and of different natures. Cells 

 which contain only fluid are rare (fat-cells, blood-cells, cells of the chorda 

 dorsalis), and it is colorless or reddish ; in general they contain, in 

 addition, corpuscles in greater or less number (elementary granules, 

 elementary vesicles, perhaps crystals), and in fact, as a rule, young cells 

 possess few, while older ones contain many, which are very often more 

 densely grouped round the nucleus, or occupy only a single spot (colored 

 nerve-cells). 



The chemical composition of the cells is as yet very obscure. The 

 contents in most cells present certain generally disseminated substances, 

 which occur dissolved in the nutritive fluid or cytoblasteina, as water, 

 albumen, fat, extractive matter, salts ; a nitrogenous substance, which 

 is precipitated by water and by dilute acids, thus resembling mucus, is 

 very extensively distributed, and considerably impedes the microscopical 

 analysis of the cells and tissues, inasmuch as it causes them to be ob- 

 scure and granular, instead of clear and transparent. Many cells con- 

 tain yet other compounds, as those of the liver, of the kidneys, of the 

 blood, &c. The cell-membrane consists of a nitrogenous substance, 

 which is unquestionably a protein compound in young cells, as we may 

 conclude from its solubility in acetic acid (partly even in the cold) and 

 in dilute caustic alkalies. Subsequently the membrane in many cells, 

 yet by no means in all (e. g. not in the blood-corpuscles, in the deepest 

 cells of the epidermis and epithelium, nor in the cells of the glandular 

 follicles), becomes less soluble, and here and there more or less approxi- 

 mates the substance of the elastic tissue. 



The cell-nucleus is a globular or lenticular, clear, or yellowish body, 

 which in the mean measures 0*002-0*004 of a line, and rarely, as in 

 the ganglion-globules and ova, attains a diameter of 0-01-0-04 of a 

 line. All nuclei are vesicles, as Schwann supposed and as I have 

 recognized to be their original and universal structure in embryos 

 and adult animals. The membrane is very delicate in the smaller 

 ones, appearing as a simple, fine, dark line ; in the larger it is more 

 marked, even of measurable thickness and limited by a double contour, 

 as in the nuclei of the ganglion-globules, of ova and of many embryos. 

 The contents of the nuclear vesicle consist, excepting the nucleolus, 



