THE CELL DOCTRINE. 35 



latter is most frequently the case in the thick strata 

 which occur in the formation of the majority of ani- 

 mal cells. We cannot, at this period, distinguish a 

 cell cavity and cell wall. The deposition of new 

 molecules between those already existing proceeds, 

 however, and is so effected that when the stratum is 

 thin, the entire layer, and when it is thick, only the 

 external portion, becomes gradually consolidated into 

 a membrane. The external portion of the layer may 

 become consolidated soon after it is defined on the 

 outside; but, generally the membrane does not be- 

 come perceptible until a later period, when it is 

 thicker and more defined internally; many cells, 

 however, do not exhibit any appearance of the for- 

 mation of a cell membrane, but they seem to be solid, 

 and all that can be remarked is that the external por- 

 tion of the layer is somewhat more compact.* 



"Immediately that the cell membrane has become 

 consolidated, its expansion proceeds as the result of 

 the progressive reception of new molecules between 

 the existing ones; that is to say, by virtue of a growth 

 by intussusception, while at the same time it becomes 

 separated from the cell nucleus The in- 

 terspace between the cell membrane and the cell 

 nucleus is at the same time filled with fluid, and this 

 constitutes the cell contents. During this expansion 

 the nucleus remains attached to a spot on the internal 

 surface of the cell membrane." Though, according to 

 Schwann, in animal cells the nucleus is never covered 

 by a lamella passing over its inner surface, as is the 



* Schwann, op. citat, p. 176. 



