32 GENERAL ANATOMY OF THE TISSUES. 



concerned in the formation of cells is so far peculiar, that — 

 1, it never produces geometrical solids, but even in the nucleus 

 and nucleolus determines the globular form ; 2, that it aggre- 

 gates, not homogeneous but chemically different substances, 

 as those which constitute the nucleus and the cell-membrane ; 

 3, lastly, that without exception, in the development of the 

 cell-membrane it limits itself, and does not, like the crystal- 

 lising force, repeatedly apply layer upon layer. Of these 

 differences, the two latter might perhaps be set aside, if with 

 regard to the second point, it were assumed that the nuclei at 

 first consist of the same substance as the cell-membranes, or 

 •are almost identical with them in chemical composition ; or if 

 we referred to the fact that in crystallisation also, different 

 substances may unite into one crystal, or that a substance, b, 

 may crystallise around a substance a. 



In order to diminish the force of the third fact adduced (this 

 objection, indeed, does not hold with regard to endogenous deve- 

 lopment, and therefore in almost all plants, since it is impossible 

 here that the cells should produce any more layers around them- 

 selves), it might be urged that the permeability of the organic 

 membranes, the exchange of constituents which takes place 

 between the juices of the cell and the cytoblastema, and the ap- 

 plication of the molecules attracted from the cytoblastema to the 

 growth of the membrane, and to precipitates within the interior, 

 are perhaps the reasons why the cells develope no new circum- 

 ferential layers. It is not necessary to carry out this last 

 possibility any further, nor to bring forward the difficulties 

 which are opposed to this view also, — among which not the 

 smallest is, that the organic development of vesicles does not 

 stop at the formation of nuclei, but is only finished with the 

 completion of the cell membrane ; since in any case the facts 

 brought forward are more than enough to demonstrate the 

 insufficiency of Schwann's hypothesis. I do not see, however, 

 anything better or more positive to substitute in its place, and 

 I therefore think it will be most expedient simply to group 

 together the ascertained facts into a few general propositions, 

 which may, perhaps, be done as follows. 



1. The nucleus of the cell arises in the first place, as a 

 precipitate in an organisable fluid, and afterwards becomes 

 consolidated in such a manner, that a special investment and 



