SURVEY OF CELL-LIFE. 177 



is that the external portion of the layer is somewhat more 

 compact. 



Immediately that the ccll-membranc has become consoli- 

 dated, 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. We 

 may therefore conclude that the deposition of the new mole- 

 cules takes place more vigorously between those which lie side 

 by side upon the surface of the membrane, than it does between 

 those which lie one upon another in its thickness. The inter- 

 space between the cell-membrane and cell-nucleus is at the 

 same time filled with fluid, and this constitutes the cell-con- 

 tents. During this expansion the nucleus remains attached 

 to a spot on the internal surface of the cell-membrane. If the 

 entire stratum, in which the formation of the cell commenced, 

 have become consolidated into a cell-membrane, the nucleus 

 must lie free upon the cell-wall ; but if only the external por- 

 tion of the stratum have become consolidated, the nucleus must 

 remain surrounded by the internal part, and adherent to a spot 

 upon the internal surface of the cell-membrane. It w r ould seem 

 that the portion of the stratum which remains may be disposed 

 of in two ways : either it is dissolved and forms a part of the 

 cell-contents, in which case the nucleus will lie free upon the 

 cell- wall as before ; or it gradually becomes condensed into a 

 substance similar to the cell-membrane, and then the nucleus 

 appears to lie in the thickness of the cell-wall. This explains 

 the variety in the position of the nucleus with respect to the 

 ccll-membranc. According to Schleiden, it sometimes lies in 

 the thickness of the membrane in plants, so that its internal 

 surface, which is directed towards the cell-cavitv, is covered 

 by a lamella of the cell-walJ. In animals it also sometimes 

 appears to be slightly sunk in the cell-membrane j but I have 

 never observed a lamella passing over its inner surface ; on the 

 contrary, in almost all instances it lies quite free, adherent 

 only to the internal surface of the cell-membrane. 



The particular stage of development of the nucleus at which 

 the cell commences to be formed around it varies very much. 

 In some instances the nucleus has already become a distinct 



VI 



