66 PRACTICAL ZOOLOGY. 



of the body as an accompaniment of activity, and the 

 throwing off of the waste products, are included 

 under the term nutrition. 



5. The amoeba breathes; that is, takes in oxygen and 

 throws off carbon dioxide ; this is really a part of 

 the process of nutrition, being most intimately con- 

 nected with the breaking down of the protoplasm 

 (which invariably accompanies any form of action), 

 during which not only motion, but also heat is 

 produced. 



6. The amoeba reproduces its kind by simple division 

 into two. Each individual of the higher, many-celled 

 animals develops from an egg, which is a cell, and is 

 essentially like an amoeba. As it develops, this cell 

 divides, forming two ; each of these divides, and the 

 division continues till many cells are formed; these 

 cells are at first all alike, but soon they grow different 

 from one another, and are arranged in order, forming 

 the various parts of the body ; thus one set of cells 

 form muscle, another set, developing in a different 

 way, produce the brain and nerves; other groups of 

 cells form bone, skin, cartilage, etc. Each of these 

 sets of different kinds of cells is called a tissue ; thus 

 there is muscular tissue, nervous tissue, etc. The 

 process of "growing unlike " or " growing different," 

 which the cells undergo in their development, is called 

 differentiation ; thus, of the cells which were all es- 

 sentially alike, those of one set have taken one shape, 

 and have acquired certain peculiarities by which they 

 may be recognized, and we say they have become 

 differentiated into nervous tissue or muscular tissue, 

 etc. 



