92 COMPARATIVE PHYSIOLOGY 



passing through the membranes of the digestive tract until 

 broken down into diffusible products by hydrolysis. In the 

 mammal there appear to be three stages in the process. The 

 first takes place through the agency of the gastric enzyme 

 pepsin which exerts its optimum efficiency in an acid medium 

 which is provided for by the presence of free HCl in the secre- 

 tion of the gastric mucosa. Prolonged digestion in vitro of 

 proteins in the presence of pepsin does not carry the process 

 to the liberation of amino-acids, which are the end-products 

 of protein hydrolysis in presence of inorganic catalysts. In 

 the body peptic digestion probably promotes only the initial 

 stages of splitting into simpler proteins such as proteoses and 

 peptones. 



The enzyme trypsin which is supplied by the pancreatic 

 juice can bring about the complete hydrolysis of proteins in 

 vitro. In the body it seems probable that the reaction is not 

 carried beyond the production of the relatively simple and 

 diffusible condensation-products of amino acids known as 

 polypeptides. The final resolution of these into simple amino- 

 acids is apparently effected with the co-operation of a pro- 

 teoclastic ferment in the secretion of the intestinal mucosa 

 (erepsin). Proteoclastic enzymes have been detected in 

 extracts of the digestive glands of all groups in the animal 

 kingdom. How far they are identical with those which 

 occur in the mammalian gut is not certain. Using the facility 

 with which a gelatine mixture solidifies when cooled for a 

 fixed period in the ice-bath as a measure of the progress of 

 protein hydrolysis, Bodansky and Rose (1922) extracted from 

 the mesenteric filaments of the jelly-fish Stomolophus and the 

 siphons of Physalia (Siphonophora) a digestive fluid with two 

 pH. optima at 3*0 and 7*3 respectively, roughly corresponding 

 to the pH. optima for mammalian pepsin and trypsin. A 

 rennet-like ferment capable of coagulating the milk protein 

 caseinogen was also found to be present. Yonge (1924) was 

 unable to find a pepsin- like enzyme in the digestive gland 

 of the lobster ; but free amino-acids were obtained from an 

 alkaline digest with the extract. 



In the tissues, especially in the liver of the Vertebrates, 



