70 



MANUAL OF HISTOLOGY. 



much smaller, even down to 0'006 mm., and less, whilsb other cells again 



possess nuclei whose diameter may reach 

 0-023-0-045 mm. The position of this 

 nucleus in the animal cell is at one time 

 central, at another excentric. 



The fundamental form of the object 

 under consideration, as it is met with in 

 the earliest formative cells of embryonic 

 tissue, and frequently enough in those 

 of more mature parts of the body, appears 



nucleus'atd'^Th'e nuclei themselves"lie to be that of a Vesicular body approach- 

 excentricintnebodyof thecell,*. ing ^ spherical figure (fig> ^ ^ ^ 



more or less fluid, and, it may be added, homogeneous, transparent con- 

 tents and strong cortex, which latter shows, under the strongest micro- 

 scopes of the present day, a double contour as optical 

 expression of its thickness. Thus we see that the nucleus 

 possesses an analogous structure to the cell, which is 

 endowed with a membrane, and one of whose components 

 it is. 



In the interior of this hollow nucleus, or, as it has been 

 named, nuclear utricle, or vesicle, may be discovered, single 

 or double, a roundish formation, almost a mere speck on 

 account of its minuteness: this is the nucleolus already 

 mentioned (d d). 



This fundamental form, however, of the nucleus is fre- 

 quently enough exchanged subsequently for another, alter- 

 Fig.56. TWO ceils ing thus its original appearauce, although the variations 

 muscieTi?"!the of tne nucleus may be stated as a rule to be less in propor- 

 rod-iike homoge- tion than those of the cell itself. We find, for instance, 

 cleL in turning to the consideration of some of these changes 

 of figure, that it may become elongated, as in those cells which enter into 

 the composition of unstriped muscle (fig. 56, It), or diskoid, as seen in the 

 tissue of nail (fig. 57). Ramifications of nuclei have also been met with 

 in certain cells of lower organism, but not as yet in those of the human 

 body. 



Fig. 57. Cells of nail 

 tissue, a a, view from 

 above, with the granu- 

 lar nucleus; b b, side 

 view of the cell, with 

 the flattened levelled 

 nucleus. 



Fig. 58. Flat epithelial cells, with 

 completely homogeneous smooth- 

 edged nuclei. 



On the other hand, the nucleus may exchange the original vesicular 

 condition of an earlier period for solid contents, as is the case, for instance, 

 in the superficial epithelial cells of the mouth (fig. 58), or for perfect 

 homogeneity, so that even the envelope of the nucleus is no longer to be 



