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 lens 

 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 mesh work (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. 



