150 THE NATURE AND ORIGIN OF LIFE 



less thick layer of dead matter, often very rigid and highly 

 mineralized. This protective wrapper formed of not-living 

 substances is called the shell. 



The cell-membrane as we have described it a peripheral 

 layer of the cytoplasm is never wanting. Some authors 

 speak of unicellular beings without the membrane ; but 

 they must be understood as meaning only this the peri- 

 pheral layer of the cytoplasm, though necessarily existing, 

 does not take from its contact with the environment a very 

 different appearance from that of the cytoplasm itself, not 

 enough to deserve special mention when described. 



Oftenest, when greatly magnified, the cytoplasm shows 

 a rather complex structure. We distinguish in it certain 

 more fluid parts filling the meshes of a network formed of 

 the less fluid parts. What we have said of physiological 

 division of labour helps us to understand that this must be 

 so. The cytoplasm has a veritable histology of its own, but 

 it is not easy to study. 



Haeckel has described, under the name of monera, cells 

 without a nucleus and reduced to a homogeneous cytoplasm. 

 But progress in our methods of observation hardly seems to 

 warrant faith in these monera. 



In other species, on the contrary, there has been de- 

 scribed a nucleus so voluminous that it may be considered as 

 filling the whole cell ; but here, too, there are remains of a 

 cytoplasm. 



On the side of dimensions there are tremendous differences 

 in the various unicellular species. Certain rhizopods are 

 large enough to be seen by the naked eye. There are 

 micrococci so small that, even under the strongest lenses, 

 they are seen only as geometrical points ; and even then 

 they have to be strongly coloured by suitable reagents. 



For some years back there has even been current men- 



