184 THE CELL-THEORY 



way the nucleus and later the cell are laid down as concentric 

 precipitates from the cytoblastem. Cell-formation also 

 involves the second or metabolic force, by means of which 

 the cell alters the chemical composition of the medium 

 surrounding it so as to prepare it for assimilation. Schwann's 

 attractive force brings about the actual taking up of the 

 prepared substance ; his metabolic force is the cause of the 

 digestion of food substances, and is nearly identical with 

 enzyme action. With what inorganic process, he now asks 

 (p. 239), can the process of cell-formation be most nearly 

 compared, and the answer obviously is, with the process of 

 crystallisation. Cells are, it is true, quite different in shape 

 and consistency from crystals, and they grow by intussuscep- 

 tion, not by apposition their plastic or attractive forces seem 

 therefore to be different. A still more important difference 

 is that the metabolic force is peculiar to the cell Yet there 

 are important analogies between, crystals and cells. They 

 agree in the important respect that they both grow in 

 solutions at the cost of the dissolved .substance, according to 

 definite laws, and develop a definite and characteristic shape. 

 It might even be maintained, Schwann thinks, that the 

 attractive force of crystals is really identical with that of cells, 

 and that the difference in result is due merely to the difference 

 between the substance of the cell and the substance of the 

 crystal. He points out how organic bodies are remarkable 

 for their powers of imbibition, and he seeks to show that the 

 cell is the form under which a body capable of imbibition 

 must necessarily crystallise, and that the organism is an 

 aggregate of such imbibition-crystals. The analogy between 

 crystallisation and cell-formation he works out in the 

 following manner : " The substance of which cells are 

 composed possesses the power of chemically transforming the 

 substance with which it is in immediate contact, in somewhat 

 the same way as the well-known preparation of platinum 

 changes alcohol into acetic acid. Each part of the cell 

 possesses this property. If now the cytoblastem is altered 

 by an already formed cell in such a way that a substance is 

 formed that cannot become part of the cell, it crystallises out 

 first as the nucleolus of a new cell. This in its turn alters 

 the composition of the cytoblastem. A part of the trans- 



