no THE BIOLOGY OF BIRDS 



the two muscular layers, Birds and Reptiles resemble one 

 another. 



The cloaca or terminal chamber is vaguely divided into 

 three regions : — 



(a) That which receives the rectum, 



(b) That which receives the ureters from the kidneys and 

 the reproductive ducts, and X 



(c) The terminal portion. 



From the dorsal side of the terminal portion of the 

 cloaca there grows out a rather enigmatical hollow pouch 

 called the Bursa Fab rich. Its development has been 

 followed by LeHevre and Retterer (1910) and Jolly (191 1). 

 The internal epithelial lining gives off buds which penetrate 

 into the envelope of the pouch. These buds or " closed 

 follicles " become the seat of the formation of numerous 

 white blood-corpuscles and lymph-corpuscles. Gradually, 

 however, the blood-forming ceases ; the buds are changed 

 into reticulated tissue ; the adult organ is practically a 

 retrograded mass of fibrous connective tissue. It may 

 become quite hard. 



§ 4. Physiological Summary 



Looking backwards, let us think of the whole matter 

 physiologically. The food consists of carbohydrates, fats, 

 proteins, and " accessory substances " or vitamines. It is 

 used for growth, for repair, for storage purposes, and as a 

 fuel affording energy for work. There is no particular form 

 in which proteins can be stored except in so far as they 

 are incorporated into the living tissues of the body. The 

 storable material consists of carbohydrates and fats. The 

 proteins are broken down into amino-acids (with formation 

 of nitrogenous waste) and these are used for growth, repair, 

 and combustion. Digestion is the process of dissolving 

 the food, making it more diffusible and available. The 

 solid carbohydrates are changed into fluid sugar ; the fats 

 are split up into fatty acids and glycerol ; the proteins are 

 changed into peptones. , The ptyalin or amylase of the 



