LIVING .SUBSTANCE 



79 



cell. Only a very few varieties of cells having a compact proto- 

 plasmic body reach a diameter of a few millimetres, and these 

 are amoeboid, their surface changing 

 continually and their substance being 

 in constant streaming motion. The fact 

 that compact cells whose radii are ap- 

 proximately equal in all dimensions and 

 whose protoplasm is not constantly 

 streaming never surpass the size of a 

 few millimetres, has only apparent ex- 

 ceptions. The bird's egg might be re- 

 garded as an exception. It is well 

 known that the egg of a fowl before it 

 has left the body represents a single 

 cell; an ostrich egg would, therefore, be 

 a single, compact giant cell, which ap- 

 parently would contradict the above 

 rule. This exception, however, as has 

 been said, is only apparent, for the really 

 active or living protoplasm of the egg- 

 cell has a very small bulk and in the 

 form of an extremely thin and delicate 

 lamella is laid over the rest of the 

 mass, which latter consists of inactive 



egg-yolk, the food-material for the fur- 

 ther developing and reproducing cell. 

 Hence there is here, not a solid com- 

 pact mass of living substance, but 



merely a thin lamella. Such an ex- 

 tension in one or two dimensions exists 



also in all other cells that exceed the 



usual size — e.g., the cross-striated 



muscle-cells of the leg-muscles, which 



are often more than a decimetre in 



length ; ganglion-cells, which are ex- 

 tended into nerve-fibres more than a 



metre long, and the leaf-shaped cells of 



Caulerpa. In all these cases it appears 



that the ratio of the mass to the surface 



of the cell never exceeds a certain value. 



As will be seen later, this phenomenon 



is deeply grounded in the nature of 



living substance, and the formation of a large and massive organism 



is possible only by the employment of very small autonomous 



elements, such as the cells. 



Fig. 21. — a, A smooth muscle-cell. 

 (After Schieiferdeeker.) 6. Sper- 

 matozoon of Salaraandra mact'.- 

 (ata. (After Hertwig.) k, Head; 

 .^p, tip ; rii, middle-piece ; y, un- 

 dulating membrane ; ej, end- 

 thread. 



