CELLS. 49 



into inner and outer parts, envelop, and contents, or whether they 

 are not, from the first, vesicular, cannot at present be decided. 



The nuclei being once formed, the cell-membranes are developed 

 around them, though not always in the same way. In the first place 

 they may be applied directly around the nucleus, so that the nascent 

 cell is but little larger than its nucleus ; or, in the second place, the 

 latter may become surrounded by a greater or smaller quantity of solidi- 

 fying cytoblastema, and it is only around this enveloping mass, which 

 I have called an investing globule, that a membrane forms. This last 

 occurrence in the free cell-formation has hitherto been observed only 

 in the ovum, in which the germinal vesicle, i. e. the nucleus of the egg- 

 cell, being first formed, surrounds itself with some yelk before the vitel- 

 lary membrane appears. On the other hand, cell-development directly 

 round the nucleus takes place in all the other localities which have been 

 mentioned above, and is demonstrated by the occurrence, among free 

 nuclei and large cells, of very small cells, which closely invest the nu- 

 cleus or are but little separated from it. It may, however, be remarked, 

 that perhaps in these cases also, the cell-membranes at their origin 

 are separated from the nuclei by a very small quantity of cytoblastema, 

 so small as to be incapable of detection. 



Free cell-formation is exceedingly frequent in pathological produc- 

 tions, and the cells in pus and in exudations of all kinds arise in this 

 manner ; in fact, all pathological cell-formation properly comes under 

 this head. Usually the cell-membranes here arise directly round the 

 nucleus, less commonly as it would seem round investing globules. 

 With regard to physiological processes, as has been already shown, free 

 cell-development has been much too readily taken for granted; and 

 especially as regards the epithelial and horny tissues, as well as in many 

 glandular secretions, it has been assumed without any sufficient grounds. 

 Botany knows no free cell-development.* 



11. The development of cells within other cells, or their endoge- 

 nous origin, is of very frequent occurrence, and easy to be observed 

 in embryos. The commonest form of this cell genesis is, that a so-called 

 parent cell produces two secondary cells, which from the first wholly fill 



* [There cannot be said to be any evidence of the occurrence of free cell-development in 

 animals, so long as in any case cited it is not shown that the first-formed particles which 

 make their appearance cannot have derived their origin from pre-existing formed particles, 

 cither by the detachment or fission of the latter. Not only does this condition remain unful- 

 filled for all the instances cited, but it has not been attempted, and would seem to be im- 

 possible. In pathological exudations, for instance, who shall determine that the first struc- 

 tural elements which appear, granules, free " nuclei," exudation corpuscles, &c., are not 

 directly derived either from the blood, or from the tissue into which the exudation has taken 

 place? TRS.] 



4 



