42 ZOOLOGY. 



fresh-water animals) come to be so universally distributed. 

 The power of encystment affords them a means of distribution. 

 The formation of a cyst also affords an example of the pro- 

 cess known as secretion. The cyst is not protoplasm, but it 

 is formed from protoplasm, and there can be no doubt that 

 it is formed by the modification of the katabolism of the 

 protoplasm. This is the only plain case, in amoeba, in 

 which a product of katabolism serves a useful purpose. 



7. Cells. The body of amoeba consists of a single 

 cell. This term (which is far from self-explanatory and 

 originated in quite a mistaken idea) is applied to any mass 

 of protoplasm containing a nucleus (or the essential chemical 

 substance of a nucleus). All living beings are primarily 

 composed of cells. Some consist of one cell only, as 

 amoeba ; but all the higher animals are built up of many 

 cells, and not of cells only, but of cell-products as well. 

 Protoplasm must not be thought of as a definite chemical 

 compound : it is rather to be compared to the molten mass 

 in a furnace, into which ore, flux and fuel are being thrown 

 while metal and slag are being withdrawn. We can, 

 however, recognize its general constitution. Water forms 

 more than half of it by weight ; the rest consists mainly of 

 various proteids, while other organic compounds and inor- 

 ganic salts are present in smaller quantities. It must 

 be remembered that protoplasm has always mixed in with 

 it compounds that are in process of assimilation (anastases), 

 and compounds resulting from its katabolism (katastases). 

 No chemical analysis can be made of protoplasm without 

 including these. 



The nucleus is usually spherical in shape. It is less 

 fluid than the rest of the protoplasm, containing less water 

 and more proteid, but the chief chemical difference is the 

 presence of a substance containing a large proportion of 

 phosphorus. One peculiarity results from this chemical 

 difference, which is of great advantage in microscopic 

 study : the nucleus takes up staining reagents (carmine, 

 hsematoxylin, etc.) to a much greater extent than the 

 protoplasm. 



The only way in which protoplasm is known to be pro- 



