INCUBATION, is that instinctive process by which birds sit unremittingly upon their eggs, with the exception of short intervals employed in quest of food, until, by im parting their own vital warmth, the young acquires suffi cient strength, and breaks the shell.
Persons not conversant with natural history have believ ed, that life does not exist in the egg until the commence ment of incubation, or the application of artificial heat. The effect of either does no more, however, than unfold the latent germ. About 104 degrees of heat are requir ed to hatch the eggs of our common domestic poultry, which, in hens, must be continued during 21 days. Some times the period is longer or shorter, raoging between 19 and 23. The period required by the eggs of other birds is extremely diversified ; whence Count Morozzo has infer red, that the period of incubation is proportioned to the du ration of the life of the bird ; in the same way as he con ceives the gestation of mankind and quadrupeds is propor tioned to the length of their existence. The swan, which is supposed to live 200 years, sits 42 days ; the goose, sup posed to live 80, sits 30; while the goldfinch, whose life is restticted to 18 or 20 years, does not sit on its eggs above 14 days.
The experiments of most observers have been made on the eggs of the common hen ; but Malpighi did not confine himself to these exclusively. On inspecting an egg at the larger end, where all the changes take place, several conspicuous parts appear, as the albumen, the yolk, the air vessel, and the membranes, besides some which are less obvious ; but the highest magnifier is incapable of disco vering the germ of the Nothing more than a confused whitish spot, generally called the cicatricula, is discernible. Malpighi, indeed, who has treated this sub ject in a masterly manner, seems to affirm, that the embryo is perceptible before incubation, but it has been sensible to no other observer. He likewise thought its evolution would be seen in six hours after incubation, and has given a magnified delineation of the appearances. Haller con siders it unnecessary, at least in the climate of Switzer land, to make observations previous to seven hours ; and he remarks, that Malpighi's egg of six hours was as far advanced as his of twenty-four. On the whole, it is to be concluded, that none of the more evident changes happen before the twelfth hour. At that time a slight enlarge ment of the cicatricula is found to have taken place. The embryo, still of the most irregular and indistinct figure, has undergone some expansion : it is surrounded by a line of a greenish or deep yellow colour. The rings or ha lones environing it, of which sometimes three and some times four arc risible, seem better defined. No alteration is yet perceptible on the bulk or consistence of the fluids, though hailer says, that an egg, with seven hours incuba tion, contains a greater quantity of albumen than it will ever have afterwards. A cavity at the large end, called the follicultzs orris, is full of atmospheric air, which is gra dually enlarged by incubation ; and although twenty-one eggs, newly laid, afford only a cubic inch of air, the same quantity is yielded by two after twenty days.
Considerable alteration will be found to have taken place in twelve hours more, or after the interval of a day, from the commencement of incubation. The egg floats with the small end downwards, by the enlargement of the cavity, or escape of its own fluids, and the figure of the embryo is better defined. It seems about .18 of an inch in length, whereas in twelve hours it does not exceed .1. The cir cles or ,halones are more extended, and the interior one has acquired some resemblance to the figure of a heart, sur rounded by a line of a greenish colour. Green specks are
likewise perceptible in different parts of the area enclosed by it, which are the rudiments of blood vessels. Twelve hours later, the greenish line has gained more evidently the appearance of a vessel, and some of the specks are seen to contain blood. They are distinct blood vessels in two hours longer, and convey red blood in six more. In 48 hours, the foetus betrays signs of motion. Its head re sembles a hammer, and the tail is extremely slender. The vascular area has attained the figure of a heart, and pul sation is now evident, or perhaps some hours earlier. Still " it appeared like a cloud, and melted under the scalpel." At this period also the eyes become visible ; and the contents of the yolk bag have grown somewhat more fluid. It was only at this time that Haller succeeded in measuring the length of the embryo, which proved .22 of an inch in one egg, and .30 in another. It must not be sup posed, that there is the most distant resemblance between this shapeless body and a bird ; on the contrary, no two ob jects can be more dissimilar. At the end of 70 hours, the rudiments of the wings, and sometimes those of the legs, may be traced ; and at the close of the third day, or 72 hours, the changes are still more conspicuous, and the parts better defined. Before the fourth has come to a conclusion, the pulsations of the heart are so tremulously rapid, that the eye cannot follow them. But as the life of the embryo is enfeebled by exposure to the air, they become retarded, and are seen to follow each other in regular succession. The liver is perceptible before the fifth day, not of its ultimate hue and aspect indeed, but as a viscous cloud of a yellow colour ; and the eye also ap pears black ; the limbs seem to move against its termina tion. The heart gains its proper formation, and the vis cera are unfolded. On the sixth day life is so vigorous, that it is not uncommon to find the chicken lying on its back, instead of on its left side as before. The natural figure is in a considerable degree acquired ; and most of the organs exhibit a certain definite formation. The weight of the egg has now greatly diminished, the exter nal colour darkens, and the substance of the shell becomes more brittle. Next day the white is smaller, and has sunk towards the small end cf the egg, while the dimensions of the yolk are enlarged. From this period, the size of the animal advances with great rapidity. Its members are gradually unfolded, and its strength progressively increas ed. The muscular nature of the stomach is declared on the eleventh day. On the thirteenth the albumen is sect, to have attained a greenish hue, and to have acquired great er consistence ; the chicken has gained a more perfect form; its head is greatly curved, and the bill lies under the right wing : It is pointed, and is sometimes opened and closed. On the nineteenth, half the egg is occupied by air, and the chicken is heard to try. In the next place, the shell becomes extremely fragile, and towards the close of the twentieth day is generally cracked, but not, as erro neously supposed, by the mother. After this the chicken is always heard, its beak projects, and, by the full period of incubation being completed, it is liberated from its con finement on the twenty-first. Now the albumen has to tally disappeared, and the contents of the yolk bag occupy the stomach, which is observed to be greatly distended. See Harveii Opera. Malpighi De Formatione Pzilli in Ovo. Appendix de Ovo Incubato. 1Iaitrejean Observations sur la Formation du Coeur dens le Poulet. Haller Sur la For mation du Coeur dans le Poulet. Journal de Physique, vol. liv. p. 480. Transactions of the Linneatz Society, vol. x. Rees' Cyclop cedia, voce Incubation. (c)