254 CLASSICS OF MODERN SCIENCE 

 more simple law, by which several layers are formed and the molecules 

 are not all deposited between those already present, cannot yet be ex- 

 plained. The formation of layers may be repeated once, twice, or 

 thrice. The growth of the separate layers is regulated by a law, 

 that the deposition of new molecules should be greatest at the part 

 where the nutrient fluid is most concentrated. Hence the outer part 

 particularly becomes condensed into a membrance both in the layer 

 corresponding to the nucleus and in that answering to the cell, be- 

 cause the nutrient fluid penetrates from without, and consequently is 

 more concentrated at the outer than at the inner part of each layer. 

 For the same reason the nucleus grows rapidly, so long as the layer 

 of the cell is not formed around it, but it either stops growing alto- 

 gether, or at least grows much more slowly as soon as the cell-layer 

 has surrounded it ; because then the latter receives the nutrient matter 

 first, and, therefore, in a more concentrated form. And hence the 

 cell becomes, in a general sense, much more completely developed, 

 while the nucleus-layer usually remains at a stage of development, in 

 which the cell-layer had been in its earlier period. The addition of 

 new molecules is so arranged that the layers increase more consider- 

 ably in superficial extent than in thickness; and thus an intervening 

 space is formed between each layer and the one preceding it, by 

 which cells and nuclei are formed into actual hollow vesicles. From 

 this it may be inferred that the deposition of new molecules is more 

 active between those which lie side by side along the surface of 

 the membrane, than between those which lie one upon the other in 

 its thickness. Were it otherwise, each layer would increase in thick- 

 ness, but there would be no intervening cavity between it and the 

 previous one, there would be no vesicles, but a solid body composed 

 of layers. 



Attractive power is exerted in all the solid parts of the cell. This 

 follows, not only from the fact that new molecules may be deposited 

 everywhere between those already present, but also from the forma- 

 tion of secondary deposits. When the cavity of a cell is once formed, 

 material may be also attracted from its contents and deposited in 

 layers ; and as this deposition takes place upon the inner surface of the 

 membrane of the cell, it is probably that which exerts the attractive 

 influence. This formation of layers on the inner surface of the cell- 

 membrane is, perhaps, merely a repetition of the same process by 



