PROPERTIES PECULIAR TO LIVING MATTER 167 



unite with that of the other, giving a large fusion protoplast 

 which develops a heavy protect! ve wall and is a sexually formed 

 spore. It is called a zygospore (meaning yoked spore) because 

 the gametes are similar, like the halves of a yoke. This cell 

 union is the same in all essentials, including the final fusion of 

 the two nuclei, as the fertilization of the egg, except that the 

 two sexual cells, or gametes, are not different in form as are 

 eggs and sperms (examine illustrations of Volvox (Fig. 178), 

 CEdogonium (Fig. 182), Fucus (Fig. 199), etc.). The fruiting of 

 Spirogyra is a relatively simple illustration of a sexual process, 

 for the gametes are similar and have never become differen- 

 tiated into eggs and sperms. 



Development proceeds through continued cell divisions, which 

 lead to growth and a gradual specialization or setting apart of 

 certain cells for particular kinds of work in the body. This 

 specialization of cells results in the various forms of cell struc- 

 tures, or tissues, of the mature organism. So the life history is a 



o u 



succession of cell divisions, and the reproduction of the species 

 is a return to a one-celled condition through the reproductive 

 cells (gametes and spores). The animal and plant body dies, 

 but the stream of life flows on through the reproductive cells. 

 This is the outline of the cell theory of organization, which 

 perhaps ranks next to the theory of organic evolution as one of 

 the fundamental principles of biological science. 



201. Properties peculiar to living matter. We have noted 

 that the chemical composition and reactions of protoplasm are 

 exceedingly complex, but nevertheless there are no reasons for 

 supposing that they are outside of chemical and physical laws. 

 However, protoplasm has properties which distinguish it from 

 lifeless matter (see also Sees. 45-47). 



Protoplasm has the power of growth and repair. This means 

 that protoplasm can manufacture living substance out of the 

 lifeless and add the same to itself. It can replace with new 

 and fresh living matter the waste material which is used up 

 or discarded during the life processes. 



