BACTERIAL INVASION 227 



a fashion that an effective barrier to generalized infec- 

 tion results. 



This conclusion may appear paradoxical, but it is 

 no more so than the well established relationship be- 

 tween toxins and the formation of their specific anti- 

 toxins. 



The colon of higher animals always contains large 

 numbers of various bacteria; indeed, it has been esti- 

 mated that from 5 to 20 per cent of the dry weight of 

 the fecal mass consists of bacteria. Under normal 

 conditions the bacteria present in the colon do not 

 appear to cause the host difficulties of any sort. Oc- 

 casionally, however, these organisms do gain access 

 to the body and infection results. Many years ago the 

 writer had occasion to investigate this matter. It was 

 found, by the microscopic examination of appropri- 

 ately stained sections of the intestinal tract of animals 

 which had been fed cultures of B. colt, that the bac- 

 teria may diffuse into the intestinal wall, passing be- 

 tween the epithelial cells and between the muscle cells 

 of the muscular mucosa into the areola tissue. (Turck, 

 1909, 1910.) Later it was found that this process could 

 be followed very easily in fetal animals after injection 

 of bacterial cultures into the intestine. (Turck, 1908, 

 1914.) 



In all instances it was observed that as the diffusing 

 bacteria entered the submucous zone rapid bacterioly- 

 sis took place. In consequence, this zone of bacterial 

 destruction was named the "Zona Transformans." 

 Presumably it is the zone which normally protects an 

 animal from infection by the organisms present in the 

 intestinal tract. If for any reason the Zona Trans- 

 formans ceases to function in its normal capacity, then 



