OVUM. 



61 



men 58, the yolk 31}, and the shell with its 

 lining membrane, 10. 



When eggs are kept exposed, they gradually 

 sustain a small loss, due chiefly to the eva- 

 poration of water, and amounting to about one 

 grain per day. When putrefaction ensues, an 

 additional loss from chemical changes occurs. 



During incubation, the loss of weight is 

 more considerable, amounting in twenty-one 

 days to 16 or 17 per cent., or nearly one 

 sixth of their entire substance.* The loss 

 by an egg during incubation, therefore, is 

 eight times as great as that which occurs in 

 an egg kept at the usual atmospheric tem- 

 perature for the same period a circumstance 

 which depends partly on the higher tem- 

 perature, but principally on the evolution of 

 carbon from the oily matters of the incubated 

 egg, in combination with the oxygen of the 

 air, or as carbonic acid, &c. 



Of the 17 parts per cent, lost during incu- 

 bation, not more than 5 or 6 consist of water, 

 and the remaining two thirds, that is 10 or 

 1 1 parts, are derived from the oily and other 

 substances of the egg which undergo chemical 

 changes attendant upon the process of orga- 

 nisation and respiration of the embryo. 



By evaporation to dryness of the whole 

 egg without the shell and membrane, about 

 27 per cent, of the substance are left; the 

 oily ingredients of this residue, amounting to 

 about lOf, are almost all contained in the 

 yolk, and the remaining 16J parts of solid 

 matter are nearly equally divided between the 

 yolk and the white. The yolk, therefore, is 

 much richer in the fixed and solid parts than 

 the white ; but its specific gravity, as will 

 afterwards be seen, is considerably reduced 

 by the larger quantity of oily matter it con- 

 tains : the per-centage of solid matter (inde- 

 pendently of the oleaginous substance) con- 

 tained in the yolk and albumen, is in the pro- 

 portion of 32 in the first to 14- in the second.f 



The solid residue obtained by evaporation 

 of the white at a low temperature, amounting 

 to nearly one seventh of the whole, consists 

 chiefly of albumen ; along with which there 

 is also some animal matter which has hitherto 

 been named by chemists as extractive, and a 

 small amount of salts, which are principally 

 alkaline sulphates, muriates and phosphates, 

 with phosphate of lime, some free soda and 

 sulphur. 



The yolk contains little more than half its 

 weight of water, or 54 per cent. The remain- 

 ing 46 parts consist of about 17 of albumen, or 

 analogous principles, 28 of oily matter, and 1 

 of salts. These last are chiefly alkaline mu- 

 riates and sulphates, phosphate of lime and 

 magnesia, and traces of iron, sulphur, and 



* See Prout, On the fixed Principles of the Egg, 

 Philos. Trans, and Annals of Pliilos. for 1822. Also, 

 by the same author, On the Changes of the Egg in 

 Incubation, in the same Journal, for 1823. ; and, 

 Paris, On the Physiology of the Egg, in Linnean 

 Soc. Trans, vol. x. p. 304, and Annals of Philos. 

 1821. 



f .See Prevost and Morin, in Journ. de Pharmacie 

 for 1846, and Sacc, in the Eggs of the Bantam 

 Fowl, in Annal. cles Scien. Nat. for 1847, p. 69. 



phosphorus. The albumen has an alkaline, 

 the yolk a neutral, reaction.* 



The membrane lining the shell consists 

 apparently of a protein compound, analogous 

 somewhat to that of the elastic yellow tissue. 



The shell consists of earthy salts deposited 

 in a delicate matrix of animal matter, which 

 last constitutes not more than 3 per cent, 

 of the whole. The earthy ingredients are 

 in great part carbonate of lime, together with 

 a little carbonate of magnesia, and phosphate 

 of lime and magnesia. 



Of the ingredients of the egg before men- 

 tioned, the albumen and other animal prin- 

 ciples, together with the sulphur and salts, 

 are no doubt more immediately employed in 

 the growth of the embryo; while the oily 

 matter, besides contributing, as it appears, in 

 some part, to the same purpose, serves more 

 directly and in greater quantity for the re- 

 spiratory process, in which it is consumed 

 largely during incubation. 



The alkalinity of the white of egg appears 

 to depend on the presence of caustic soda, 

 which albumen has the property of separating 

 from its carbonate. 



The following tabular view exhibits in a 

 general way the change in the relative pro- 

 portion of the ingredients of the egg resulting 

 from incubation f : 



Shell and membrane 



Albumen, &c. - 



Oily matter, &c. 



Water - - - - 



T /Water - 5-6") 



ljOSS t Oily matt.,&c. 11-4 J 



When an egg is examined immediately on 

 being laid and while yet warm, or still better 

 when taken from the egg-bag of the fowl pre- 

 vious to laying, the yolk and white fill com- 

 pletely the interior ; but immediately on 

 cooling, a small space or vacuity appears ge- 

 nerally towards the obtuse end of the egg, and 

 this air-space increases gradually in size as the 

 eggs are longer kept and the natural evapora- 

 tion of water proceeds. This space is formed 

 by the separation of the two principal layers 

 of the lining membrane of the shell. Durin^ 

 incubation the air-space increases much more 

 rapidly; and indeed towards the end of this 



* Composition of the yolk, according to Gobley, 

 in Journal de Pharmacie, 8e. se'r. torn. ix. p. 174. 

 Water, about - 53' 



Vitelline, albumen, and protein com- 

 pounds - - - 1C -5 



Oily matters 29- 



Salts, &c. - 1-5 



100-0 



These salts are the following viz., chloride of 

 sodium and potassium, sulphate of potassa, muriate 

 of ammonia, phosphates of lime and magnesia, 

 lactic, acid, colouring matter, iron, 

 t From Sacc, loc. citat. 



