70 J. PERCY BAUMfeERGER 



upon the microorganisms in the decay with which they infest 

 the plant. 



Since Darwin's work ('81) it has been assumed that the earth- 

 worm finds its food in the humus of the soil it infests. Humus 

 soil is notably rich in microorganisms, for these are the elabora- 

 tors of humus from plant and animal remains, and it is possible 

 that they are of food value to the worm. 



Such structures as the endostyle and dorsal pharyngeal groove 

 of Amphioxus and the Tunicates are probably for the purpose of 

 entrapping microphytes of various kinds. 



As pointed out by Osborne and Mendel ('14 b), microorganisms 

 may also be of value to higher animals as elaborators of protein 

 in the digestive tract from the non-protein substances ingested. 

 This would be especially true in herbivorous animals, as Armsby 

 ('11) has shown that non-protein substances are a source of pro- 

 tein in these animals, probably due to the formation of digestible 

 bacterial protein in the digestive tract. . The possibihty that the 

 flora of the digestive tract may modify the food elements supplied 

 in a nutritional experiment is a drawback to the use of mammals 

 in such experiments. An insect like Drosophila should be of 

 value as material for such experiments because of the ease with 

 which it is sterihzed. 



Many attempts have been made to rear mammals under sterile 

 conditions, but most of these have failed so that it has been a 

 great question whether or not it is possible for animals to live in 

 the complete absence of microorganisms. As a large flora nor- 

 mally occurs in the digestive tract, it was necessary to sterilize 

 the animal before it had taken food and to keep it in a sterile en- 

 vironment, therefore Pasteur ('85) suggested the use of hens, the 

 «ggs being well fitted for sterilization. Pasteur's suggestion was 

 later carried out, but the first experiments were made by Nuttall 

 and Thierfelder ('95-'96) on guinea-pigs, the young being re- 

 moved aseptically from the mother by cesarian section. The ani- 

 mals were kept in a complicated aseptic environment and were 

 fed upon food of animal origin. The animals gained 10 grams 

 in one week (84 grams total weight) and appeared to be normal 

 on the eighth day, when the experiment had to be discontinued. 



