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 bodii 

 of mammalia are developed from nucleated cells. 



T/i e 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- 

 00030 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 cg^ 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. If the 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 verv 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- 



