46 GENERAL ANATOMY OF THE TISSUES. 



almost invariably of a pellucid or slightly yellowish never more darkly 

 tinged fluid, in which water and acetic acid precipitate the same dark 

 granules, as in the cells, for which reason, the nuclei never preserve their 

 natural homogeneous clear appearance, when examined according to the 

 ordinary methods. More rarely the nuclei have formed contents, as 

 the spermatic filament in the semen ; in ova peculiar granules, the so- 

 called germinal spots ; also in the fat of Piscicola (Leydig). In respect 

 of their chemical composition, only this much can be said of the nuclei, 

 that their membranes are nitrogenous, and in general but little different 

 from the substance forming the younger cell-membranes; they are, how- 

 ever, dissolved more slowly in alkalies, and are but slightly attacked by 

 dilute acetic and mineral acids. In the latter circumstance they ap- 

 proximate the elastic tissue, from which, however, they are most essen- 

 tially distinguished by their easy solubility in alkalies. 



The nuclei are found, so far as I have observed, in all cells of embryos 

 without exception, and in those of adults, so long as the cells are still 

 young. In general only a single nucleus exists in each cell, except 

 when it is multiplying; in the latter case, however, two or more nuclei 

 arise, according to the number of the developing cells. In certain cells 

 we meet with more numerous nuclei ; thus, in those of the semen, 4, 

 10, 20, and more; also in the substantia grisea centralis of the spinal 

 cord, of the supra-renal capsules, of the pituitary body, in the hepatic 

 cells of embryos, in the foetal medullary cells of bone, and elsewhere. 

 That nuclei also occur free, and take part in the formation of certain 

 tissues, has already been stated. 



The nucleoli are round, sharply defined, generally dark, fat-like gra- 

 nules, which, on the average, measure 0*001-0-0015 of a line, are often 

 almost immeasurably small, and in embryos, in the germinal vesicles of 

 ova as the germinal spots, and in the ganglion-globules, attain the size 

 of O'003-O'Ol of a line. In all probability they are always vesicular, 

 as may be surmised from their sharply circumscribed form, their simi- 

 larity to elementary vesicles, and also from the circumstance that in 

 certain cells, especially in ova and ganglion-globules, a larger or smaller 

 cavity filled with fluid frequently becomes developed in them. The 

 chemical composition of the nucleoli is unknown : their external 

 appearance, their similarity to the elementary vesicles, their disap- 

 pearance in caustic alkalies, and their insolubility in acetic acid, 

 would lead us to ^suppose them to be fat ; the membranes may, as in 

 the elementary vesicles, be a protein compound. Nucleoli are found 

 in the great majority of nuclei, so long as these are still young, and 

 in many during their whole existence ; but nuclei also exist, in which 

 nucleoli cannot be recognized with certainty, or at least become ob- 

 vious only at a later period ; and therefore, at present, the nucleolus 

 cannot be so unconditionally recognized to be an essential constituent 



