THE PHYSIOLOGY OF EXCRETION. 85 



part of the figure represents the appearance of the living cell, 

 the lower part that of a stained section. 



In the living excretophore we find a great quantity of yellow 

 drops with a dark line around each drop. This is only an optical 

 phenomenon due to the strong refracting index of this sub- 

 stance. Similar black rings surround, e.g., fat drops. In the 

 middle of each drop we notice a number of small, dark granules. 

 In a preparation stained with iron-haematoxylin these drops 

 appear homogeneous and a little irregular in shape, which latter 

 is due to influence of fixing reagents. The cytoplasmic network 

 is very fine and seems to be in some places broken and destroyed. 

 The nucleus shows no features of especial interest. The yellow 

 drops (i) are but isolating fluid, secreted around the small excre- 

 tory granules. I regard this fluid as a mixture of some albumi- 

 nous substance and some oil or fat acid. This conclusion I am 

 forced to make by the fact that these drops slowly stain brown 

 in the living cell after the addition of osmic acid to the water 

 on the slide. I think that they are only partly composed of oil 

 or fat, because if they consisted of pure oil or pure fat they 

 would darken rapidly with osmic acid, which is not the case. 

 It is apparent that the secretion of these isolating drops must 

 cause great changes in the chemical structure of the cytoplasm, 

 and this consideration will give us a key to answer the next 

 question : Why do the excretophores wander towards the dorsal 

 surface of the animal after they ae crowded with waste 

 products ? 



The answer to this question is brief and, in fact, is already 

 contained in the preceding speculations: We have seen that a 

 great amount of the oxygen in the cytoplasm has been used up 

 in the formation of passive structures (the isolating drops), and 

 that in order to keep up its motion and activity the cell has to 

 make up for this loss of oxygen. It is, therefore, necessary for 

 the excretophore to wander towards a place in the organism 

 where free oxygen is continually present. 



The dorsal layers of the skin are the seat of capillary respira- 

 tion in these animals, and in this place fresh oxygen is continu- 

 ally absorbed. This oxygen exercises a chemotropic influence 

 upon the excretophores, which strive to reach the seat of 



