128 OF THE STRUCTURAL ELEMENTS OF THE HUMAN BODY. 



the tissues are produced and increased ; for it has been hitherto chiefly observed 

 in cases in which a plastic exudation has been poured out for reparative purposes, 

 or in which a structure of an abnormal character is being generated. The first step 

 in the process usually appears to be the aggregation of some of the minute mole- 

 cules which the fluid or semi-solid blastema contains, so that they form little 

 rounded masses, or nuclei, from which the new cells originate. The mode in 

 which the cell-wall is formed does not appear to be by any means constant. 

 Sometimes it seems to rise and separate itself from the nucleus itself, as if it 

 were formed by the melting together of molecules precipitated upon or attracted 

 to the nucleus ; but more frequently it appears to be generated by the expansion 

 of the wall of the nucleus itself; in either case, however, commencing to enlarge 

 and separate itself from the nucleus by the endosmosis or assimilation of fluid 

 from the surrounding blastema. 1 But there are other cases in which the nuclear 

 particles appear to draw around them certain components of the substance in 

 which they lie, before any cell-wall can be discerned, this being subsequently 

 generated around this collection, which then constitutes the contents of the 

 newly-formed cell. This first-formed nucleus may be persistent, and may take 

 a part in the subsequent vital actions of the cell, of whatever kind these may 

 be ; or it may be superseded by a second nucleus, which subsequently makes its 

 appearance in some other part of the cell-wall. It seems probable that the first 

 formation of the chyle and lymph-corpuscles, which subsequently develop them- 

 selves into blood-corpuscles, takes place from free nuclei ; and it has been main- 

 tained by some that the epidermis and epithelium are likewise formed in the 

 same manner; both these views, however, require confirmation. 



107. Another mode of Cytogenesis must be mentioned as of occasional occur- 

 rence ; namely, the expansion of a single apparently homogeneous granule into 

 a cell, without the previous formation of any perceptible nucleus; a distinction 

 first showing itself between the exterior portion of the granule, which becomes 

 the cell-wall, and the interior which seems to melt down to form the first cell- 

 contents; and the former extending itself, as the latter augment by imbibition 

 from the surrounding fluid. It is on this plan that the development of the 

 " zoospores" of many of the Algae seems frequently to take place ; a for these, at 

 the time of their emission, can frequently not be considered as anything else 

 than " granules." And the first development of blood-corpuscles in the Chick, 

 as seen in blood drawn from the heart on the third day, is affirmed by Mr. Ma- 

 cleod 3 to present a similar series of phenomena. Indeed, it may be surmised 

 that, when a large number of young cells are simultaneously or successively 

 evolved from the nucleus of a parent-cell ( 105), it is by some such mode of 

 development from the individual particles composing the nucleus ; for the appear- 

 ance which is presented when this process is being carried on, is very much that 

 of the sprouting of its marginal granules into young cells (Fig. 9). And it 

 seems probable that, wherever a single granule is thus evolved into a cell, it has 

 been discharged as a reproductive particle from a pre-existing cell ; its conditions 

 of development being in this respect different from those of the granules which 

 form nuclei by their aggregation in the midst of a plastic exudation, and which, 

 not having undergone the same preparation, are not endued with vital properties 

 of as high a character. On the whole of this subject, however, considerable 

 obscurity still rests; and while it is next to certain that cells may be generated 

 within the Human body in each of the modes now described, there is still very 

 much to be learned respecting the conditions under which every form of the 

 process, respectively occurs. 



See Prof. Bennett's Treatise "On Cancerous and Cancroid Growths," p. 146. 



2 See "Prin. of Phys., Gen. and Comp.," \\ 142, 267, a, Am. Ed. 



3 " Lond. and Edinb. Monthly Journ.," Sept., 1842. 



