Some experiments on the changes which take place in the fixed principles of the egg during incubation

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, second, and third weeks of incubation, his experiments being principally 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 considerable 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 incineration, in consequence of the phosphorus which it contains burning into phosphoric acid, which forms a coating that protects the coal from the action of the air. The general results of these experiments are thrown into the form of tables. They show that the relative weights of the constituent principles of different eggs vary considerably, 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 precluded 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 the source of the lime, though he by no means intends to assert the impossibility of its being derived from the shell.