THE PHYSIOLOGY OF EXCRETION. 89 



to trace the course of the carmine particles within the tissues 

 of the animals. At various periods I made teased preparations 

 of the living tissues, and detected the carmine particles in the 

 excretophores and in their disintegration stages up to the true 

 pigment patches. They are only discernible under very high 

 powers, as they are not larger than microsomes, a fact which 

 makes me think that nutrition and excretion are not merely 

 chemical but also mechanical processes, perhaps rudely com- 

 parable to a filtering process. The small black dots in Figs, i 

 and 2 represent the carmine particles {car.). 



We come now to the second part of our task, to the fate of 

 the intra-coelomatic excretophores. The first question we have 

 to answer here is: Why do the excretophores gather arotmd the 

 7ieph7'idial funnels after they have become loaded zuith waste 

 products ? I have come to the conclusion that the funnel cells 

 of the leeches possess a different chemical constitution from 

 the other nephridial cells, for the following reason: It is a fact 

 that they stain much deeper than the nephridial cells, and that 

 they retain the stain with greater tenacity ; indeed, in order 

 to get a good stain of the funnel cells with iron haematoxylin, 

 it is necessary to decolorize the sections until the nephridial 

 cells have become nearly colorless. To what is this due .'' 



It is highly probable that the great activity which the funnel 

 cells possess, the constant contact with substances which are 

 chemically entirely different from cytoplasm, must be the cause 

 of very active metabolic processes within the cells. In this great 

 chemical activity of the cell numerous waste products, both 

 solid and liquid, must be formed and expelled from the cell. 

 I ascribe the great affinity to stains to the presence of such 

 particular waste products. We know that bases will intensify 

 the color of haematoxylin, whereas acids will weaken it. I 

 assume, therefore, a basic nature for these secretions or 

 excretions. 



It is a well-known fact that certain micro-organisms possess 

 a strong chemical affinity to certain chemical substances, a 

 phenomenon which we call positive chemotropism. Thus the 

 spermatozoa of ferns exhibit strong positive chemotropism 

 towards malic acid in a certain concentration. If one puts a 



