CELLS. 17 



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 ac- 

 cording 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 cells 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-mem- 

 branes; they are, however, dissolved more slowly in alkalies, 

 and are but slightly attacked by dilute acetic and mineral 

 acids. In the latter circumstance they approximate the elastic 

 tissue, from which, however, they are most essentially distin- 

 guished 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 granules, which, on the average, measure 0001 — 00015"', 

 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'OOo — 001'". In all 

 probability they are always vesicular, as may be surmised 

 from their sharply-circumscribed form, their similarity to 

 elementary vesicles, and also from the circumstance that in 

 certain cells, especially in ova aud ganglion-globules, a larger 

 or smaller cavity filled with fluid frequently becomes deve- 

 loped in them. The chemical composition of the nucleoli 



i. 2 



