122 SIMPLE HISTOLOGICAL ELEMENTS. 



formation ; though here, also, it may still be a question whether the 

 nucleus itself becomes the future cell, or whether it forms the new 

 cell around itself, as is generally asserted. 1 



If, in any case, but a single generation of cells is required, for 

 the reason that the original ones are to remain unchanged in the 

 organism, no nuclei are, of course, needed, if we still maintain that 

 the nucleus exists merely to develop another cell. Some have 

 placed the fat-cells in this category, though incorrectly, as will 

 appear. (Chap. Y.) 



In the ovum alone (so far as is known) a layer of semi-solid 

 plasma is interposed between the nucleus and the cell-membrane, 

 and entirely incloses the former. (Fig. 55.) 



In pathological productions, free cell-development is very common. 

 Pus-cells and exudation-cells (or corpuscles) are thus formed. This 

 method of development of epithelial cells, and of those of which 

 nails and horn are formed, is, however, rather assumed by authors 

 than established by proof. 



II. Development of Cells from pre-existing Cells. 

 A. Endogenous Cell-development. :\, 



The most frequent form of endogenous cell-development is that 

 in which two secondary cells are produced within the original one. 

 Here, in the first place, the nucleus of the parent cell enlarges, and 

 exhibits two nucleoli ; then it becomes elongated, and constricted 

 in the middle, and at last separates into two portions. Each of 

 these becomes the nucleus of a new cell, and the two nucleated cells 

 thus formed at length fill up the interior of the parent cell. (Figs. 

 60 and 61.) The parent cell may burst and set free the secondary 

 cells, or continue to enlarge till several generations have been thus 

 developed within it. On the other hand, it may coalesce with the 

 substance which unites the cells as a matrix. 



This mode of development in persistent parent cells 2 occurs in the 

 cartilages of all young animals, and very probably during the de- 

 velopment of the tissues generally, in their embryonic state. (Kbl) 



Thus the nucleus existing in a cell is the germ which is to be 

 developed into the cell of the next generation that of which the 



1 The blind adherence hitherto to the observations upon cell-development, of 

 Schwann and Schleiden, has retarded the progress of histological science, and tbe 

 whole subject demands investigation de novo. 



2 Cavities, rather. See Chap. VI. 



