ON CELLS, ETC. 37 



form presented by the cells, for they may be flattened even to 

 the total disappearance of the cavity, or elongated into cylinders 

 and fibres. From these circumstances, many of the cells which 

 now come before us for consideration, have been described as 

 mere globules, or granules, terms which do not express their 

 true signification, and even when they were spoken of as cells, 

 or cells furnished with a nucleus, the description rested only 

 upon a slight analogy, since but very i'cw of them (for example, 

 the pigment-cells), were proved to be actually hollow cells. 

 But — as the precise signification of the nucleus is unknown, and 

 as the cell-membrane is not proved to be anything essential to 

 those cells (and this follows from their accordance with vege- 

 table cells), upon the analogy with which the proof of the 

 cellular nature of the rest of the globules provided with a 

 nucleus will be based, — there is no contradiction involved in the 

 supposition that a nucleus may be contained in a solid globule 

 as well as in a cell. 



From the difficulties of this investigation above detailed, it 

 will be seen that a given object may really be a cell, when even 

 the common characteristics of that structure, namely, the per- 

 ceptibility of the cell-membrane, and the flowing out of the cell- 

 contents, cannot be brought under observation. The possibility 

 that an object may be a cell, does not, however, advance us 

 much ; the presence of positive characteristics is necessary in 

 order to enable us to regard it as such. In many instances 

 these difficulties do not present themselves, and the cellular 

 nature of the object is immediately recognized ; in others, the 

 impediments are not so great but that the distinction between 

 cell-membrane and cell-contents is at least indicated, and in 

 such cases other circumstances may advance that supposition 

 to a certainty. The most important and abundant proof as to 

 the existence of a cell is the presence or absence of the nucleus. 

 Its sharp outline and dark colour render it in most instances 

 easily perceptible ; its characteristic figure, especially when it 

 encloses nucleoli, and remarkable position in the globule under 

 examination, (being within it, but eccentrical, and separated 

 from the surface only by the thickness of the assumed cell-wall,) 

 all combine to prove it the cell-nucleus, and render its analogy 

 with the nucleus of the young cells contained in cartilage, and 

 with those of vegetables, as also the analogy between tli- 



