CELLS. 31 



§ 13. 



Theory of Cell-development. — Among the few hypotheses 

 which have, up to the present time, been proposed in expla- 

 nation of the development of cells, that of Schwann, who com- 

 pares it with the formation of crystals, is certainly the most 

 attractive. "Without overlooking the differences between a 

 crystal and a cell, which chiefly consist in the former being 

 solid and homogeneous, in its growing by apposition, and in 

 its being bounded by plane surfaces and angles, Schwann 

 endeavours to explain cell-development as a crystallisation 

 of organic matter, and to deduce from the permeability of the 

 latter the differences in the phenomena presented by the 

 two. In a fluid containing organic matters dissolved in con- 

 siderable proportions, a granule, the nucleolus, is precipitated. 

 Once formed, this attracts nutriment from the cvtoblastema. 

 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 for- 

 mation 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. 1 

 On the other hand, it is evidently going too far to call cell- 

 development simply a crystallisation of permeable organic sub- 

 stances, since in this case important differences are overlooked, 

 and non-essentials are made unduly prominent. For it must 

 not be forgotten, that organic permeable substances also crystal- 

 lise; that, in fact, if Reichert have observed correctly (M tiller'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 



1 [See note, p. 25. — Eds.] 



