74 ZOOLOGY 



evidently must be in segments and jointed. The articulation, or 

 joint, is as important for motion as the harder parts. Clearly 

 the muscles, to do any work on the skeleton, must attach on 

 opposite sides of the joint. 



99. Growth. — There are no special organs of growth, yet 

 growth is one of the most immediate and important manifesta- 

 tions of the nutritive process. Growth is to be defined as in- 

 crease in volume or mass and may result from either or all of 

 three processes: viz. (i) absorption of water, (2) formation of 

 new protoplasm and the multiplication of cells, and (3) forma- 

 tion of non-protoplasmic cell-products, either within or among 

 the cells. 



The rate and character of growth are modified by such ex- 

 ternal conditions as temperature, light, quantity and quality 

 of the food supply, etc. Growth does not continue indefinitely. 

 Its continuance is determined by the relation of the anabolic 

 to the katabolic processes in the body. The time comes in the 

 life of every complex organism where the income no longer 

 exceeds the outgo, and growth must cease. Later still the wear 

 is not made good by the income, and death results. Just what 

 causes organisms to cease growing we do not know. 



100. Reproduction and the Reproductive Organs. — Since 

 individual organisms are limited both with regard to growth 

 and length of life, it is apparent that a given species of animals 

 cannot continue, unless some method of originating new in- 

 dividuals be found. This production of new individuals by 

 the instrumentality of the old is reproduction. In many of 

 the lower animals this is merely a growth process, — "growth be- 

 yond the limits of the individual." In the single-celled ani- 

 mals reproduction means the separation of the protoplasm into 

 two or more masses, by dividing into two equal parts (fission), 

 by breaking into a large number of sub-equal portions (spore 

 formation), or by budding (Chapter III, §40). In budding 

 there is the formation of a local outgrowth which ultimately 

 attains the size and character of the parent. In division the 

 resulting individuals cannot be distinguished as parent and 

 offspring. They are both offspring. Such reproduction, in- 



