80 BIOLOGICAL LECTURES. 



the intestine respiration and assimilation, we might as well 

 call the crushing of ore in a quartz mill gold or silver 

 production. 



Excretion itself is not the mere throwing out of waste 

 products, but it is a complicated cellular process, a task which a 

 certain cell fulfils as an independent unit as well as in intimate 

 correlation with other elements of the body. My attempt is 

 to show the minute mechanism of excretion, and I shall try to 

 make this clear by tracing the paths along which the waste 

 products are carried, by describing the changes which take 

 place during excretion in the cellular elements involved in this 

 process, and by showing how the balance between expended 

 energy and regeneration is continually kept up by the introduc- 

 tion of new structures and new actions in the relative elements. 



Cellular metabolism shows the following processes, which, by 

 the way, are so interwoven and gradational in their mutual 

 relations that it is very hard to keep them apart : 



(1) Assimilation, or the transformation of given nutritious 

 substances into bioplasm with the aid of preexistent bioplasm. 



(2) Respiration, or the oxidation of bioplasm, by which the 

 bioplasm is partly transformed into energy, motion, and heat, 

 and partly changed into oxidized materials, waste products. 



(3) The process of excretion, or the discharge of waste prod- 

 ucts from the cell. 



All three (assimilation, or building up; respiration, or trans- 

 formation ; and excretion, or loss) together constitute the 

 cellular activity which is itself the answer to outer stimuli. 



The external stimuli and the stimulated cell, or bioplasm, 

 together constitute life; the phenomena of life are stimulus 

 and reaction (cellular activity), and what we call structure is 

 only the path along which reaction follows the stimulus. This 

 I shall briefly attempt to show in this paper, but I have to 

 refer for a more detailed account of my theories to a paper 

 which is being published in Germany vtit\\\z&Hirudineenstudien, 

 and which will possibly appear at the same time as this paper. 1 



1 A short abstract of these views is contained in my paper on the individuality 

 of the cell (State of New York State Hospitals Bulletin, vol. II, No. 2, 1897), 

 which paper appeared nearly one year after this lecture was delivered. 



