20 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



As was said before, animal protoplasm is pre-eminently katabolic, and the 

 evidence of its katabolism is found in the waste products, such as CO 2 , H 2 O, 

 and urea, which are given off from animal organisms. Assimilation and dis- 

 assimilation, or anabolism and katabolism, go hand in hand and together 

 constitute an ever-recurring cycle of activity which persists as long as the 

 material retains its living structure and which as a whole is designated under 

 the name metabolism. In most forms of living matter metabolism is in some 

 way self-limited, so that gradually it becomes less perfect, old age comes on, 

 and finally death ensues. It has been asserted that originally the metabolic 

 activity of protoplasm was self-perpetuating that, barring accident, the cycle 

 of changes would go on forever. Resting upon this assumption it has been 

 suggested by Weissmann that the protoplasm of the reproductive elements 

 still retains this primitive and perfect metabolism and thus provides for the 

 continuity of life. The speculations bearing upon this point will be discussed 

 in more detail in the section on Reproduction. 



Reproduction in some form is also practically a universal property of 

 living matter. The unit of structure among living organisms is the cell. 

 Under proper conditions of nourishment the cell may undergo separation into 

 two daughter cells. In some cases the separation takes place by a simple act 

 of fission, in other cases the division is indirect and involves a number of 

 interesting changes in the structure of the nucleus and the protoplasm of the 

 body of the cell, or cytoplasm, as it is frequently called. In the latter case 

 the process is spoken of as karyokinesis or mitosis. This act of division was 

 supposed formerly to be under the control of the nucleus of the cell, but 

 modern histology has shown that in karyokinetic division the process, in 

 many cases at least, is initiated by a special structure to which the name cen- 

 trosome has been given. The many-celled animals arise by successive divi- 

 sions of a primitive cell, the ovum, and in the higher forms of life the ovum 

 requires to be fertilized by union with a spermatozoon before cell-division 

 becomes possible. The sperm-cell acts as a stimulus to the egg-cell (see section 

 on Reproduction) and rapid cell-division is the result of their union. It must 

 be noted also that the term reproduction includes the power of hereditary 

 transmission. The daughter-cells are similar in form to the parent-cell and 

 the organism produced from a fertilized ovum is substantially a facsimile of 

 the parent forms. Living matter, therefore, not only exhibits the power of 

 separating off other units of living matter, but of transmitting to its progeny 

 its own peculiar internal structure and properties. 



Contractility and conductivity are properties exhibited in one form or 

 another in all animal organisms and we must believe that they are to be 

 counted among the primitive properties of protoplasm. The power of con- 

 tracting or shortening is, in fact, one of the commonly recognized features of 

 a living thing. It is generally present in the simplest forms of animal as 

 well as vegetable life, although in the more specialized forms it is found for 

 the most part only in animal organisms. The opinion seems to be general 

 among physiologists that wherever this property is exhibited, whether in the 



