CELLS. 57 



in considerable proportions, a granule, the nucleolus, is precipitated. 

 Once formed, this attracts nutriment from the cytoblastema, and thus 

 becomes the nucleus, which Schwann considers to be solid. This still 

 goes on attracting to itself a substance, which, becoming more and more 

 condensed, at last forms a membrane ; which, allowing the passage of 

 fluid cytoblastema through its pores, becomes detached from the nucleus, 

 and we have a cell. In this exposition, we must admire not only the 

 skilful, acute working out of the fundamental idea which the original 

 treatise manifests, but also the assumption of a molecular attraction in 

 cell-development, analogous to that which occurs in the formation of 

 crystals, for the existence of which there is, in fact, decisive evidence 

 (only in part, however, known to Schwann), such as the action of the 

 nuclei in the cleavage process, in cell-division, in cell-development, round 

 portions of contents, in the cyclosis, and in the formation of granular 

 precipitates in cells.* On the other hand, it is evidently going too far 

 to call cell-development simply a crystallization of permeable organic 

 substances, since in this case important differences are overlooked, and 

 non-essentials are made unduly prominent. For it must not be forgot- 

 ten that organic permeable substances also crystallize ; that, in fact, if 

 Reichert have observed correctly (MUller's " Archiv," 1849), and I see 

 no reason for doubt, histogenetic substances capable of forming tissues, as 

 albumen for instance, assume a crystalline form. Hence the molecular 

 attraction 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 aggregates, not homogeneous but 

 chemically different substances, as those which constitute the nucleus 

 and the cell-membrane ; 3, lastly, that without exception, in the develop- 

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

 lizing 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 sub- 

 stance as the cell-membranes, or are almost identical with them in 

 chemical composition ; or if we referred to the fact that in crystalliza- 

 tion also, different substances may unite into one crystal, or that a 

 substance, b, may crystallize round a substance, a. 



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

 tion, indeed, does not hold with regard to endogenous development, and 

 therefore in almost all plants, since it is impossible here that the cells 

 should produce any more layers around themselves), it might be urged 

 that the permeability of the organic membranes, the exchange of con- 

 stituents which take place between the juices of the cell and the cyto- 

 blastema, and the application of the molecules attracted from the cyto- 

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



* [See note, p. 52. — Tks.] 



