THEODOR SCHWANN 267 



made to accord with the representation of crystals capable of imbibi- 

 tion . . . And if we ascribe to a layer of a crystal capable of 

 imbibition the power of producing chemical changes in organic 

 substances, we can very well understand also the origin of secondary 

 deposits on its inner surface as they occur in cells. For if, in 

 accordance with the laws of crystallization, the lamina has become 

 expanded into a vesicle, and its cavity has become filled by imbibition 

 with a solution of organic substance, there may be materials formed by 

 means of the converting influence of the lamina, which cannot any 

 longer be held in solution. These may, then, either crystallize within 

 the vesicle, as new crystals capable of imbibition under the form of 

 cells ; or if they are allied to the substance of the vesicle, they may so 

 crystallize as to form part of the system of the vesicle itself : the lat- 

 ter may occur in two ways, the new matters may be applied to the in- 

 crease of the vesicle, or they may form new layers on its inner surface 

 from the same cause which led to the first formation of the vesicle 

 itself as a layer. In the cells of plants these secondary deposits have 

 a spiral arrangement. This is a very important fact, though the laws 

 of crystallization do not seem to account for the absolute necessity of 

 it. If, however, it could be mathematically proved from the laws of 

 the crystallization of inorganic bodies, that under the altered circum- 

 stances in which bodies capable of imbibition are placed, these deposits 

 must be arranged in spiral forms, it might be asserted without hesi- 

 tation that the plastic power of cells and the fundamental powers of 

 crystals are identical. 



We come now, however, to some peculiarities in the plastic power 

 of cells, to which we might, at first sight, scarcely expect to find any- 

 thing analogous in crystals. The attractive power of the cells mani- 

 fests a certain degree of election in its operation ; it does not attract 

 every substance present in the cytoblastema, but only particular ones ; 

 and here a muscle-cell, there a fat-cell, is generated from the same 

 fluid, the blood. Yet crystals aflford us an example of a precisely simi- 

 lar phenomenon, and one which has already been frequently adduced 

 as analogous to assimilation. If a crystal of nitre be placed in a solu- 

 tion of nitre and sulphate of soda, only the nitre crystallizes ; when a 

 crystal of sulphate of soda is put in, only the sulphate of soda crystal- 

 lizes. Here, therefore, there occurs just the same selection of the sub- 

 stance to be attracted. 



