MODES IN WHICH IT IS EFFECTED. 123 



in which the nucleus, instead of dividing into two portions, only breaks up 

 into several particles (though even this may be effected by successive dupli- 

 cations) each of which appears to possess the power of enlarging and 

 becoming vesicular ; the several minute vesicles as they increase in size 

 gradually obliterating the original nucleus, and eventually constituting the 

 chief contents of the cell. Each of these minute vesicular particles 

 probably constitutes a germ of a fresh cell, into which it is subsequently 

 developed either by simple enlargement, or by serving as a cytoblast 

 around which a cell- wall forms. In other cases, again, apparently under the 

 influence of the nucleus when present, or even independent of it, minute 

 vesicular bodies are developed within the cell itself, which by enlarging 

 they gradually fill, and eventually burst. Previous to their discharge from 

 the parent cell, or shortly afterwards, a new generation of cells is developed 

 within each of them by the same process by which they themselves had 

 been formed. Another form of cell-multiplication has been described as 

 occurring in vegetable structures, in which a cell appears to divide by 

 the formation of a partition across its cavity, whereby two new cells 

 are formed. But as explained by Schleiden, in which explanation Dr. 

 Sharpey* agrees, this apparent mode of division is probably merely an 

 instance of the endogenous production of twin cells, the contiguous sides 

 of which form the septum as in C, fig. 8, p. 71. In a few cases, again, the 

 multiplication of cells takes place by the growth of young sprouts or offshoots 

 from the parent cell. This variety, which is confined entirely to vegetable 

 structures, is well illustrated in the mode of growth of the yeast-plant, f 



4. The tendency of nearly all recent observations on the subject has 

 been to confirm the general correctness of Schwann's account of the 

 various changes which the primary cells undergo in the production of the 

 elementary tissues of the body.J In some instances, however, there is 

 sufficient evidence to shew that this account requires to be modified. This 

 seems to be especially the case in regard to the cellular, tendinous, and 

 elastic tissues, each of which was supposed by Schwann to be formed by 

 the elongation of cells and their division into bundles of fibres. But that 

 such a mode of development appears not to take place will be presently 

 shewn when considering the transformations undergone by the nuclei. 



In regard to the development of bone, a considerable amount of informa- 

 tion has been of late added to the comparatively imperfect account of it 

 furnished by Schwann. But it is considered unnecessary to enter here into 

 the details of this, since the whole of the subject has been of late so ably 

 discussed by Dr. Sharpey, in a standard work on anatomy. 



* Quain's Anatomy. Fifth edition, p. xlviii. 



f For accounts of the above modes of cell-multiplication see especially Henle, Kolliker, 

 Vogt, Reichert, and Vogel, in the works referred to. 



J For this account see Miiller's Physiology, pp. 397 and 1643. 

 Quain's Anatomy, by Dr. Sharpey and Mr. Quain, p. cxlvii. 



