176 SURVEY OF CELL-LIFE. 



molecules between the particles of this membrane takes place 

 in such a manner that more molecules are deposited between 

 those particles which lie side by side upon its surface than 

 there are between those which lie one beneath another in its 

 thickness, the expansion of the membrane must proceed more 

 vigorously than its increase in thickness, and therefore a con- 

 stantly increasing space must be formed between it and the 

 nucleolus, wherebv the latter remains adherent to one side of 

 its internal surface. 



I have made no observations on the formation of nuclei with 

 more than one nucleolus. But it is easy to comprehend 

 how it may occur, if we conceive that two nucleoli may lie 

 so close together that the layers which form around them 

 become united before they are defined externally, and that by 

 the progressive deposition of new molecules, the external limi- 

 tation is so effected that two corpuscles are enclosed by it at 

 the same time, and then the development proceeds as though 

 only one nucleolus were present. 



When the nucleus has reached a certain stage of develop- 

 ment, the cell is formed around it. The following appears to 

 be the process by which this takes place. A stratum of sub- 

 stance, which differs from the cytoblastema, is deposited upon 

 the exterior of the nucleus. (See pL III, fig. 1, d.) In the 

 first instance this stratum is not sharply defined externally, 

 but becomes so in consequence of the progressive deposition 

 of new molecules. The stratum is more or less thick, some- 

 times homogeneous, sometimes granulous ; the latter is most fre- 

 quently the case in the thick strata which occur in the forma- 

 tion of the majority of animal 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 — be- 

 comes gradually consolidated into a membrane. The external 

 portion of the layer may begin to become consolidated soon 

 after it is defined on the outside; but, generally, the membrane 

 does not become perceptible until a later period, when it is 

 thicker and more defined internally ; many cells, however, do 

 not exhibit any appearance of the formation of a cell-mem- 

 brane, but they seem to be solid, and all that can be remarked 



