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. Ifthe 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 cha- 
racteristic form be further demonstrated. On the other hand, 
that of the larger blood-corpuscles may be distinctly recog- 
nized as a cell-nucleus. 
The cell-nucleus is generally dark, granulous, often some- 
what 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 being 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- 
