10 ANIMAL FORMS 
therefore we are at present justified in defining the cell as 
a small mass of protoplasm enclosing a-nucleus. 
11. Structure of protoplasm.—When seen under a glass 
of moderate power protoplasm gives no indication of any 
definite structure, and even with the highest magnifica- 
tion it presents appearances which are not clearly under- 
stood. According to the commonly accepted view, it con- 
sists of two portions, one, the firmer, forming an excessively 
delicate meshwork (Fig. 4, A) enclosing in its cavities 
the second more fluid part. Therefore, when highly mag- 
nified, the appearance would be essentially like a sponge 
fully saturated with water; but it should be remembered 
that in the protoplasm the sponge work, and possibly the 
fluid part, is living, and that both are transparent. 
There are reasons for thinking that the structure and 
the composition of protoplasm may change somewhat under 
certain circumstances. It certainly is not everywhere alike, 
for that of one animal must differ from that of another, and 
different parts, such as the liver and brain, of the same form 
must be unlike. These differences, however, are minor 
when compared to the resemblances, for, as we shall see, 
this living substance, wherever it exists, carries on the pro- 
cesses of waste, repair, growth, sensation, contraction, and 
the reproduction of its kind. 
