EMBRYONIC FOOD OF BIRDS. 35 



of a highly combustible body for the purposes of respiration. 'Whatever, 

 therefore, is requisite for the well-being of the animal economy is present 

 in abundance in such a diet. 



From an examination of the diet-scales of the educational and invalid 

 establishments of London, the prisons and the hospitals, Beneke obtains 

 the result that the nitrogenized should be to the non-nitrogenized food in 

 weight as one to five. From other data, Frerichs calculates Ratio of nitro- 

 that the diurnal consumption should be 2.17 oz. avoirdupois non-ntoo"en- 

 of nitrogenized, and 15.54 oz. avoirdupois of non-nitrogen- izedfood. 

 ized food, that is, about as one to seven. Whatever is taken more than 

 this is superfluous. 



The peculiar advantages arising from the use of casein, which in a solu- 

 ble form possesses the quality of dissolving large quantities of phosphate 

 of lime, unquestionably determine its employment as a constituent of 

 milk. But there are circumstances under which a necessity arises for 

 the use of other nitrogenized compounds, such as albumen, in early nu- 

 trition ; and then it is remarkable by what indirect methods the difficulty 

 of its want of solvent power over that earthy body is compensated for. 

 The foetal period of the life of birds furnishes an example. In the egg 

 there is, of course, whatever is wanted for the development Development 

 of the young animal ; for, merely by the process of incuba- th* e g^ori"in 

 tion, or submitting the egg to a due temperature for a suita- of its parts. 

 ble length of time, with the access of atmospheric air, the young chicken 

 forms, with all its parts complete its bony, muscular, nervous systems, 

 feathers, beak, claws. The phosphate of lime required for the skeleton 

 is not present as such, but is formed as incubation goes on ; for in the 

 yolk there is free phosphorus, to which the air finds access through the 

 pervious shell, and, effecting its oxidation, phosphoric acid is the result. 

 This reacts on the "carbonate of lime, of which the shell consists, decom- 

 poses it, and the phosphate of lime forms. For this reason we observe, 

 as the incubation proceeds, that the shell becomes lighter and thinner. 

 The albuminous fluid which constitutes the white of the egg has little 

 power of holding bone-earth in solution ; but by manufacturing the salt 

 in this manner, as it is wanted, the development of the young bird goes 

 on without difficulty. To insure the due supply of oxygen, an air-bub- 

 ble is placed at the broad end of the egg, so that, should any transient 

 circumstance interfere with the passage of air through the pores of the 

 shell, there is a little reservoir of that material on which to rely. 



The mammalia find in milk all that they need in their infantile life 

 for their nutritive purposes. In the same manner birds, in their foetal 

 life, have whatever they require in the egg. For the former, casein is 

 the nutritive element ; for the latter, albumen. In both cases a ready 

 transmutation of that element into muscle-fibrin occurs. 



