Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/199

SURVEY OF CELL-LIFE. 173 of these cells without nuclei. Thus the mode of growth, in this instance, is similar to that of the nucleated cells, after the formation of their cell-membrane.

By far the greater portion of the animal body, at least ninety-nine hundredths of all the elementary parts of the bodies of mammalia are developed from nucleated cells.

The cell-nucleus is a corpuscle, having a very characteristic form, by which it may in general be easily recognized. It is rather round or oval, spherical or flat. In the majority of fully-developed animal cells its average size would be about 0-0020-0:0030 Paris inch; but we meet with nuclei which are very much larger, and others, again, much smaller than this. The germinal vesicle of the bird’s egg may be regarded as the largest cell-nucleus; the nuclei of the blood-corpuscles of warm-blooded animals afford examples of very small cell- nuclei. If the latter were but a very little smaller they would escape observation altogether, and the blood-corpuscles would then appear to be cells without nuclei. No other structure can be detected in these very small nuclei, nor can their characteristic form be further demonstrated. On the other hand, that of the larger blood-corpuscles may be distinctly recognized as a cell-nucleus.

The cell-nucleus is generally dark, granulous, often somewhat yellowish; but some occur which are quite pellucid and smooth. It is either solid, and composed of a more or less minutely granulated mass, or hollow. Most nuclei of animal cells exhibit more or less distinct trace of a cavity, at least, their external contour is generally somewhat darker, and the substance of the nucleus seems to be somewhat more com- pact at the circumference. The nucleus may often be traced through its progressive stages of development from a solid body to a perfect vesicle ; this may be observed in the nuclei of the cartilage-cells in the branchial cartilages of tadpoles. The membrane of the cell-nucleus and its contents may be distinguished in those which are hollow. The membrane is smooth, structureless, and never of any remarkable thickness, that of the germinal vesicle beimg the thickest. The con- tents are either very minutely granulous, especially in the small hollow cell-nuclei, or pellucid, as in the germinal vesicle, and the larger nuclei in the cells of the branchial carti-