Cassell's popular natural history

THE illustrious Harvey has said, "Every animal is born from an egg." At first hearing, this assertion might be doubted, and it might be supposed that eggs are laid only by birds; but it will be found, on limited inquiry, that fishes and reptiles increase their species by eggs also: hence they are said to be oviparous animals; the ovum, or egg, being excluded from the body entire, and subsequently hatched.

Eggs, it may be remarked, bear no proportion to the size of the animals that lay them. The living creature is the little whitish circle which may be observed on the membrane that surrounds the yolk, which is the food provided for the animal's support while confined to the shell; and, throughout the range of widely-diversified creatures, there is but very little difference discoverable as to size among these vital atoms. The difference of the eggs arises from the difference in the amount of nourishment required; this amount being strictly regulated according to the size to which it is necessary the young animal should attain before it is launched into the world.

VOL. III.

* Omne animal ex ovo.

To make our meaning clear: the crocodile is destined to attain to an enormous growth, and yet its egg is smaller than that of many birds, because its young is able to enter the water and live on its own resources before it is any larger than one of our lizards; there is, therefore, only so much food placed within the shell as will support it until it attains to that size. So, also, in the case of some very large fishes, the eggs are extremely small, because, at a very early age, the young find their subsistence in seas or rivers. But a dog-fish has need of great strength, as it is of great size when it is set at liberty; the egg, therefore, in which it is produced is large, that it may have a longer period for its development.

The eggs of birds are variously tinted and mottled, and, though not in every instance, yet in many, we can certainly see a design in the adaptation of the colours to the purpose of concealment, according to the habits of the various classes of birds. Thus, as a general rule, the eggs of birds which have their nests in dark holes, or which construct nests that almost completely exclude the light, are white; as is also the case with those birds that constantly sit on their eggs, or leave them only for a short time during the night. Eggs of a light-blue or light-green tint will also be found in nests that are otherwise well concealed; while, on the other hand, a great proportion of those nests which are in exposed situations have eggs varying in tints and spots in a remarkable degree, corresponding with the colours of external objects in their immediate neighbourhood. Thus, a dull-green colour is common in

most gallinaceous birds that form their nests in grass, and in aquatic birds among green sedges; a brighter green colour is prevalent among birds that nestle among trees and bushes; and a brown-mottled colour is found in those eggs that are deposited among furze, heath, shingle, and gray rocks and

stones.

The intensity of the instinct of hatching is well known, since it diverts the animal from one of the strongestthe pursuit of food; while we see the wisdom of the provision, knowing that a steady and high temperature is necessary to bring the offspring to maturity. It is a care added to that of nestbuilding, both tending to the same end. Yet here, as in the parental affections, there are variations and exceptions. The former are shown in the unequal perseverance of different species, under disturbance or otherwise; as, in the same species, it seems to depend on the personal character of the individual parent, familiarly differing in our domestic fowls, and, perhaps, nearly unknown to us in the wild ones. The exception familiar to us is the cuckoo; and of this anomaly no adequate solution has been offered, since this bird remains with us long enough to rear its own offspring. Yet, even that exception has its interest, as the mode adopted is equally efficacious for its designed end.

SECTION OF AN EGG READY FOR INCUBATION.

The egg of a bird is beautifully adapted to, the process it has to undergo. Thus, its oval form permits a greater part of its surface to be in contact with the warm body of the parent than any other; and within, when an egg is ready for incubation, a small spot may be observed on the yolk (1), which, when examined, is apparently a vesicle, containing fluid matter. In this the embryo swims, and is kept there to the breast of the mother-bird. Nor is this all; for, were its axis through the centre, the embryo would not move with the change in the position of the egg; but the axis being placed below the centre, it must turn round with every such change, whether the yolk be lighter or heavier than the surrounding white. Two dense cords (2), each of them strongly attached to the yolkbag, pass through the white, and are connected with the inner lining membrane at each end of the egg. A spiral filament runs through each of these cords. There are thus two fixed points on which the yolk can rotate; and the spiral filament, surrounded by thickened albumen, is, by its greater

THE PROCESS OF INCUBATION.

weight, always inclined to its lowest point. And, to perfect this beautiful arrangement, the elasticity of these spiral cords is so nicely adapted to the force with which the yolk is borne up, as to restrain it at a certain point, and allow sufficient space between the yolk and the inner lining of the shell for the white to lubricate the surface of the yolk, and thus to defend the embryo from injury.

Indescribably various are the situations in which the mother-birds pass through this interesting and remarkable process. Our chief knowledge of incubation, however, arises from the attention given to the domestic hen, and some of the results, therefore, we proceed to notice.

A few hours after the hen has brooded over the eggs, a considerable alteration takes place in them. As one of the following engravings will show, the embryo acquires length, and a blood-vessel, issuing from either side, branches into numerous smaller ones, which unite at their termination, and become a boundary on the covering of the yolk. In the centre of this net-work of vessels is the embryo, multiplying as it increases, until they nearly pervade the membrane of the yolk. It appears that when these vessels are first formed, each branch is accompanied by a vessel carrying yolk into the body of the chick, that there may be a due supply for its sustenance and growth.

[graphic][graphic][merged small][merged small][merged small][merged small]

This network may be regarded as a lung, which the embryo exhibits above it, and which is found placed in that part of the egg where the oxygen can penetrate most easily-that is, below the airchamber. It is a simple pulmonary circulation, resembling that of fishes, and lasts about two days. A new circulation then begins; the vascular network, deprived of its principal venous trunks, becomes almost unfit for the process of respiration, but its nutritive functions are more developed.

The yolk becomes flattened, and a portion of the white penetrates the yolk-bag. A vesicle is now so far formed as to have passed from the body of the chick to the lining membrane of the shell, and, as the embryo grows, so does this vesicle increase. It is full of blood-vessels-a living system of arteries and veins-carrying livid, red blood from the heart of the chick, and returning it of a bright red colour, perfectly fitted for every purpose it is designed to answer.

Even at the beginning of the third day the beating of the heart is perceptible, though no blood is visible. But only a few hours elapse, and two vesicles, containing blood, appear, one forming the left ventricle, the other the great artery. The auricle of the heart may now be observed; and the writer will never forget the pleasure with which he first saw an egg at this stage of developing the chick, exhibiting, as it did, the course of the blood to the unassisted eye.

About the sixth day, the bare wings and legs of the chick may be fully seen.

The eyes are large

« Предыдущая Продолжить »

Книги

Cassell's popular natural history, Объемы 3-4;Том 128