THE ABSORPTION OF FAT BY FRESII-WATKR MUSSELS. 79 



fat throughout the body and a heavy amount in the epithelium 

 of the part of the mantle which had been exposed to the solution 

 (Fig. 2ia, PI. III.). The control individual for this experiment 

 contained much more fat throughout the deeper body tissues 

 than the other three individuals but very little in the epithelium 

 of the ventral part of the mantle (Fig. 216, PI. III.). The 

 ventral part of the foot of the mussel which had been so treated 

 for 6 hours revealed a moderate amount of fat in the epithelium 

 but scarcely any in other parts of the foot. The fat found 

 throughout the deeper body tissues of these individuals was of 

 course previously stored fat, the mussels, as stated above, 

 having been removed from the river but a short time. The 

 point intended to be made is that a heavier loading of fat is 

 found in the epithelium which had been in contact with the fat 

 solution than in that which had not. While there is a possi- 

 bility that some solution may have been carried by ciliary action 

 up the mantle to the mouth, this probably did not occur as, in 

 sections of the intestine, absolutely no fat was found in the 

 epithelial cells. It seems highly probable that the epithelium 

 of the mantle and foot absorbed the fat directly from the solution. 

 In regard to the mechanism of absorption of the fat by either 

 the intestine or body walls little can be offered. The matter 

 is quite complicated, some of the same possibilities entering here 

 that serve to render the method of absorption of fat by the 

 mammalian intestine as yet obscure. In some of the sections, 

 especially in those cut by the freezing method, numerous drop- 

 lets which took the stains used were found closely attached to 

 the outer ends of the epithelial cells of the gills or mantle. These 

 droplets were probably the fatty acids, a small part of which were 

 present in the soap solution due to hydrolysis, by which process 

 sodium hydroxide and the fatty acids would be formed; the 

 remaining droplets were no doubt due to a slight acidity of the 

 surface of the living cells resulting from the union of carbon 

 dioxide from the cells with the water, forming carbonic acid. 

 The fat may have been taken into the cells from these droplets 

 upon the surface by phagocytic action or by solution in the 

 plasma membrane, or it may have come directly from the sodium 

 soap, the collection of the droplets upon the surface of the cells 



