175 



Some Experiments on the Changes which take place in the fixed Prin- 

 ciples of the Egg during Incubation. By William Prout, M.D. 

 F.R.S. Read June 20, 1822. [Phil. Trans. 1822,^. 377.] 



In the researches detailed in this paper,, the author's attention was 

 chiefly directed to the nature, proportions, and changes of the earthy 

 and saline substances contained in the egg, and to the source of the 

 matter constituting the skeleton of the chick. He therefore analysed 

 the egg first in its recent state, and then at the end of the first, se- 

 cond, and third weeks of incubation, his experiments being princi- 

 pally confined to the eggs of the domestic fowl. 



After some preliminary details relating to the variations in weight 

 which eggs suffer when kept, and which show that for a considerable 

 period they sustain a daily loss of about nine grains, and that the 

 relative weights of the shell, albumen, and yolk, are liable to con- 

 siderable variation, Dr. Prout proceeds to describe the manner in 

 which he conducted his analysis, especially in relation to the saline 

 principles of the yolk. This substance is remarkably difficult of in- 

 cineration, in consequence of the phosphorus which it contains burn- 

 ing into phosphoric acid, which forms a coating that protects the 

 coal from the action of the air. The general results of these experi- 

 ments are thrown into the form of tables. They show that the rela- 

 tive weights of the constituent principles of different eggs vary con- 

 siderably, and that during incubation the loss of weight exceeds by 

 about eight times that which the egg sustains by ordinary keeping. 

 That in the earlier stages of incubation an interchange of principle 

 takes place between the yolk and a portion of the albumen, which 

 passes into a substance in some respects analogous to curd of milk. 

 That as incubation proceeds, the phosphorus of the yolk becomes 

 phosphoric acid, which, united with lime, is found in the bones of the 

 chicken; which lime, Dr. Prout thinks, makes its appearance in some 

 unaccountable manner, and from some unknown source, and that it 

 does not pre-exist in any known state in the recent egg. Its only 

 possible source, observes Dr. Prout, is the shell ; but we are pre- 

 cluded ascertaining the exact quantity of lime in any particular shell 

 before and after incubation, and the application of averages cannot 

 be resorted to, in consequence of the great difference of weight in 

 the shells of different eggs. The author's reasons for doubting that 

 origin, or source of the lime, are, that the membrane in contact with 

 the shell is never vascular, and that both the albumen and the yolk 

 contain at the end of incubation a considerable quantity of earthy 

 matter, which one would suppose would have been appropriated to 

 the bone in preference to that derived from a more remote source. 

 Dr. Prout observes, that the circumstance of the shell becoming very 

 brittle towards the end of incubation, may by some be ascribed to 

 the absorption of a portion of its substance ; but this he is inclined 

 rather to refer to the heat requisite to the process of incubation, 

 which he thinks is sufficient to account for all the known changes 

 which the shell suffers. He is of opinion that great doubt hangs over 



