124 OF THE STRUCTURAL ELEMENTS OP THE HUMAN BODY. 



renders it transparent, but on the contrary brings it into greater distinctness 

 (partly by rendering the cell-wall transparent) ; it is not yet known, however, 

 what is the precise nature of its component substance. It seems to be the 

 general fact, that every Animal cell possesses a nucleus at some period or other 

 of its life; and this nucleus seems to be usually persistent (as it is capable of 

 being brought into view by reagents, when it is not otherwise apparent), so long 

 as the cell itself is undergoing developmental changes. But in the state of 

 complete development, the nucleus sometimes disappears ; thus it is normally 

 absent in the red corpuscles of the blood of Mammals, and not unfrequently in 

 fat-cells ; and it is frequently seen to present a shrunken and wasted appearance 

 in the cells of tissues which are undergoing degeneration. It will be presently 

 shown, that there is strong reason to regard the nucleus as the chief agent in 

 the vital operations of the cell ; seeing that it can exercise its functions even 

 without the development of a cell-wall to enclose the substances which it draws 

 around it, and on which it exercises its peculiar powers ( 117). Where two 

 or more nuclei are seen in a single cell (Fig. 8), they may be regarded as origi- 

 nating in the subdivision of the primordial single nucleus, and as indicating the 

 approaching subdivision of tne cell itself, or the formation of a young cell with- 

 in it. 



103. The history of the Animal cell, in its simplest form, is precisely that 

 of a Vegetable cell of the lowest kind. - Every cell lives for itself, and by itself, 

 like each of the solitary cells of the humblest Protophytes ; and if the neces- 

 sary conditions be furnished (these being essentially a due supply of nutriment, 

 and a proper temperature), it may continue to live and to grow, and may go 

 through all the phases of its development, quite independently of the organism 

 of which it originally formed part. Of this we have numerous examples in the 

 artificial implantation of parts of one body upon or within another ; the graft 

 uniting itself with its new stock, and continuing to grow after its own fashion 

 at the expense of the nourishment thence derived. But a still more remark- 

 able example is normally and constantly presented by the spermatic cells of 

 certain animals, such as the Decapod Crustacea 1 and certain Nematoid Entozoa ; 3 

 which are cast forth from the organs of the male in which they were generated, 

 and are transferred into the body of the female, when as yet they are in a com- 

 paratively early stage of their own development; the spermatozoa being not 

 yet formed within them, but being produced during the subsequent life of the 

 cells, which apparently goes on as favorably within the generative passages of 

 the female, as it would have done within the organs in which the spermatic cells 

 were at first formed, the requisite conditions being duly supplied. 3 All the 

 component cells of any one organism may be considered as the descendants of 

 the primordial cell in which it originated; but the methods of their production 

 are by no means the same in every instance, an end essentially the same being 

 brought about by means which appear (at least) to be very different. The 

 various modes of cell-development may, however, be reduced to two principal 

 forms; that, namely, in which the new cells arise from or within pre-existing 

 cells, being produced by the subdivision either of the cells themselves, or of 

 their nuclei, which is termed endogenous development; and that in which they 

 originate in germs developed de novo in the midst of an organizable " blastema," 

 which has been prepared by a previous exercise of vital force, and which still 

 requires the continued operation of that force ab extra for its due organization 

 ( 29). Each of these modes of cytogenesis, or cell-development, will now be 

 separately considered. 



' Mr. H. D. S. Goodsir, in "Anatomical and Pathological Observations," p. 39. 

 Dr. Nelson, "Proceedings of the Royal Society," J 

 3 For an application of this curious fact to the possib 

 protracted gestation in the Human subject, see CHAP. xix. 



Dr. Nelson, "Proceedings of the Royal Society," June 19, 1851. 



3 For an application of this curious fact to the possible explanation of certain cases of 



