VARIOUS OPINIONS REGARDING IT. 121 



writers on the subject, but since these have, for the most part, a tendency 

 to confirm one or other of the views stated above, it is perhaps unne- 

 cessary to do more than refer to them here.* In collecting together the 

 above facts, it has been the writer's endeavour to ascertain whether the 

 various accounts of different observers could be so far reconciled as to 

 furnish conclusions pointing to the existence of any one uniform and 

 constant plan, according to which the development of cells is in all cases 

 effected. But it will be at once evident, from what has been stated above, 

 that so far as our present data extend, no such single uniform plan can be 

 said to exist ; though it is not improbable that further investigations will 

 shortly lead to its discovery, and that then the several varieties hitherto 

 observed, may be found to be only modifications of one universal mode 

 of development. 



From what has been said above, it appears tolerably certain that cells 

 may sometimes originate independent of pre-existing nuclei, and that, 

 therefore, the views of Schleiden and Schwann in respect of the im- 

 portance of these structures in the genesis of cells, must be somewhat 

 modified. Yet it is not satisfactorily shewn that in any mode of cell- 

 formation cases ever occur in which one or more minute elementary par- 

 ticles, corresponding to the nucleoli of Schwann, do not exist previous to 

 the formation of any other part of the cell. If subsequent investigations 

 prove that the pre-existence of such particles is a circumstance of in- 

 variable occurrence, it may be reasonably inferred that they are the real 

 germs or cytoblasts from which the cells originate. When once formed, 

 these particles may give rise to the production of cells in one or other 

 of the various ways above described. Each one may either grow and be 

 itself developed into a cell by incorporating nutritive matter, and simply 

 enlarging, as is supposed by Mr. Macleod f to be the case in the develop- 

 ment of the blood-corpuscles of the chick, by Vogt J in the development 

 of the cells of the chorda dorsalis, and by Karsten in the develop- 

 ment of all varieties of cells. Or it may serve as a centre around which 

 matter is deposited to form a nucleus, from which a cell-membrane sub- 

 sequently springs in the manner maintained by Schleiden and Schwann 

 to prevail in most vegetable and animal tissues. Or, again, it may serve 

 as the true nucleus to a primary cell growing around it; and this, by 

 Kolliker, is considered to be its ordinary office. It must be mentioned, 

 also, that even a primary cell may act the part of a nucleus, so far, at 

 least, as to cause the growth around it of another secondary cell-membrane. 



* The whole subject will be found well discussed by Reichert, in his Reports in Mailer's 

 Archiv. during the last three or four years, by Kolliker in Schleiden and Naegeli's Zeit- 

 schrift, and by Henle in his late Reports in Canstatt's Jahresbericht. 



f London and Edinburgh Monthly Journal of Medical Science, Sept. 1 842. 



J Entwickelungs-geschichte der Geburtshelferkrote, p. 126. 



De Cella Vitali. Berlin, 1843. 



