38 ON CELLS AS THE BASIS 



globules under examination, in which it lies, and those cells, 

 consequently the existence of a spherical cell-membrane in the 

 globules, extremely probable. More than nine tenths of the 

 globules in question present such a nucleus ; in many the 

 special cell-membrane is indubitable, in most it is more or 

 less distinct. Under such circumstances, we may be permitted 

 to conclude that all those globules which present a nucleus of 

 the characteristic form and position, have also a cell-membrane, 

 although, from the causes before specified, it may not be per- 

 ceptible. The different tissues will also afford us many instances 

 of other circumstances which tend to prove the existence of 

 an actual cell-membrane. An example of what is referred to 

 would be afforded by an instance, in which a certain corpuscle 

 (furnished with a nucleus), about the cellular nature of which 

 a doubt existed, could be proved to be only a stage of deve- 

 lopment, or modification in form, of an indubitable cell. The 

 cell-nuclei and their distance from each other when scattered 

 in a tissue, also serve as indications, when the outlines of the 

 cells have to be sought for. They likewise serve to guide 

 conjecture as to the earlier existence of separate cells, in 

 instances where they have coalesced in the progress of develop- 

 ment. When a globule does not exhibit a nucleus during 

 any one of the stages of its development, it is either not a cell, 

 or may at least be preliminarily rejected, if there be no other 

 circumstances to prove it such. Fortunately, these cells devoid 

 of nuclei are rare. 



In addition, however, to the cellular nature of the elementary 

 structures of animal tissues, there are yet other points of 

 accordance between them and the cells of plants, which may 

 generally be shown in the progress of their development, and 

 which give increased weight to the evidence tending to prove 

 that these elementary structures are cells. The exceedingly 

 frequent, if not absolutely universal presence of the nucleus, 

 even in the latest formed cells, proves its great importance for 

 their existence. We cannot, it is true, at present assert 

 that, with regard to all cells furnished with a nucleus, the 

 latter is universally the primary and the cell the secondary 

 formation, that is to say, that in every instance the cell is 

 formed around the previously existing nucleus. It is probable, 

 however, that such is the case generally, for we not only meet 



