24 STRUCTURAL COMPOSITION OF HUMAN BODY. 



acquires, when that fluid is in contact with it, a darker and 

 more distinct outline. It is commonly, too, the part of the 

 mature cell which is capable of being stained by an ammo- 

 niacal solution of carmine the test, it may be remarked, 

 by which, according to Dr. Beale, protoplasm or germinal 

 matter may be always known. 



Nuclei may be either free or attached. Free nuclei are 

 such as either float in fluid, like those in some of the secre- 

 tions, which appear to be derived from the secreting cells 

 of the glands, or lie loosely embedded in solid substance, as 

 in the grey matter of the brain and spinal cord, and most 

 abundantly in some quickly-growing tumours. Attached 

 nuclei are either closely imbedded in homogeneous pellucid 

 substance, as in rudimental cellular tissue ; or are fixed on 

 the surface of fibres, as on those of organic muscle and 

 organic nerve-fibres ; or are enclosed in cells, or in tissues 

 formed by the extension or junction of cells. Nuclei en- 

 closed in cells appear to be attached to the inner surface of 

 the cell- wall, projecting into the cavity. Their position in 

 relation to the centre or axis of the cell is uncertain ; often 

 when the cell lies on a flat or broad surface, they appear 

 central, as in blood corpuscles, epithelium-cells, whether 

 tesselated or cylindrical; but, perhaps, more often their 

 position has no regular relation to the centre of the cell. 

 In most instances, each cell contains only a single nucleus; 

 but in cartilage, especially w r hen it is growing or ossifying, 

 two or more nuclei in each cell are common; and the 

 development of new cells is often effected, by a division or 

 multiplication of nuclei in the cavity of a parent cell ; as in 

 the primary blood-cells of the embryo in the germinal 

 vesicle, and others. 



When cells extend and coalesce, so that their walls form 

 tubes or sheaths, the nuclei commonly remain attached to 

 the inner surface of the wall. Thus they are seen imbedded 

 n the walls of the minutest capillary blood-vessels of, for 

 example, the retina and brain ; in the sarcolemma of 



