OKGANS OF GENERATION. 297 



finds, therefore, in the yolk and white of the egg an abundant quan- 

 tity of albuminous bodies of different kinds, and especially a phos- 

 phorized albumin in the yolk. Further, we also find lecithin in the 

 yolk, which seems to habitually occur in the developing cell. The 

 occurrence of glycogen is doubtful, and the carbohydrates, which 

 have no direct value as tissue-builders, are perhaps represented by a 

 very small amount of glucose. On the contrary, the egg contains 

 a large proportion of fat, which doubtless is an important source of 

 nutrition and respiration for the embryo. The cholesterin and the 

 lutein can hardly have a direct influence on the development of the 

 embryo. The egg also seems to contain the mineral bodies neces- 

 sary for the development of the young animal. The lack of phos- 

 phoric acid is compensated by an abundant amount of phosphorized 

 organic substance, and the nucleoalbumin containing iron, from 

 which the haematogen (see page 292) is formed, is doubtless, as 

 BUNGE claims, of great importance in the formation of the haemo- 

 globin containing iron. The silicic acid necessary for the develop- 

 ment of the feathers is also found in the egg. 



During the period of incubation the egg loses weight, chiefly due 

 to loss of water. The quantity of solids, especially the fat and the 

 albumins, diminishes and the egg gives off not only carbon dioxide, 

 but also, as LIEBERMANN has shown, nitrogen or a nitrogenized 

 substance. This loss is compensated by the absorption of oxygen, 

 and it is found that during incubation a respiratory exchange of 

 gas takes place. While the quantity of dry substance in the egg 

 during this period always decreases, the quantity of mineral bodies, 

 ulbumin, and fat always increases in the embryo. The increase in 

 the amount of fat in the embryo depends, according to LIEBER- 

 MANN, in great part upon a taking up of the nutritive yolk in the 

 abdominal cavity. The weight of the shell and the quantity of lime 

 salts contained therein remains unchanged during incubation. The 

 yolk and white together contain the necessary quantity of lime for 

 development. 



The most complete and careful chemical investigation on the 

 development of the embryo of the hen has been made by LIEBER- 

 MANN. From his researches we may quote the following: In the 

 earlier stages of the development, tissues very rich in water are 

 formed, but on the continuation of the development the quantity of 



