CONDITIONS OF ANIMAL LIFE 



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definite structure, and even with the highest magnification 

 it presents appearances which are not clearly understood. 

 According to the commonly accepted view, it consists of 

 two portions, one, the firmer, forming an excessively delicate 



FIG. 1. Different types of cells composing the body of a highly developed animal. 

 A, cell ; /, food materials ; n, nucleus, B, blood-cell. C, nerve-cell with small 

 part of its fiber. D, muscle fiber. E, cells lining the body cavity. F, lining of 

 the windpipe. G, section through the skin. Highly magnified. 



meshwork (Fig. 1, A) enclosing in its cavities the second 

 more fluid part. Therefore, when highly magnified, the 

 appearance would be essentially like a sponge fully satu- 

 rated 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 



