580 STRUCTURE OF THE BIRD'S EGG. own substance is scarcely distinguishable. Not only thousands but tens of thousands of eggs are often produced by a single individual, their aggregate forming what is known as the "hard roe;" whilst the "soft roe" or "milt" is the corresponding mass of sperm-cells produced by the male. 755. After the ovum of the Bird has quitted the ovarium, and is passing through the oviduct towards its outlet, it receives layer after layer of albumen poured out in a viscid condition from the lining membrane of the oviduct, forming the "white" of the egg (fig. 315, g); and this is inclosed in a, cicatricula; b, yolk-bag; c, membrane lining shell; d, attachment of chalaza; e, chalazæ; f, air-space; g, albumen. a double membrane composed of a network of fibres, which is formed by the consolidation of a plastic exudation (§ 391), poured out after the albuminous exudation has been completed. The outer layer of this membrane is consolidated by the deposit of calcareous particles in the interspaces of its fibrous matting, so as to form the "shell" of the egg; an arrangement that gives the necessary protection, without cutting-off the contents of the shell from that communication with the atmosphere which is requisite for the development of the embryo. The inner layer, which forms a lining to the shell, separates into two laminæ at the large end of the egg; and, inclosed between these, there is a bubble of air (f), which serves to give the young bird, just before it is hatched, the EARLIEST STAGES OF EMBRYONIC DEVELOPMENT. 581 of filling its lungs with air. The yolk-bag floats within power the albumen, and always tends to take the highest place, being the lighter of the two; but it is kept nearly to one place by two cords (e, e) termed the chalaza, which seem formed of peculiarly viscid albumen, and connect the yolk-bag with the lining membrane at the two ends of the shell (d, d). In this manner the yolk-bag is always kept at the part of the shell where it can most favourably receive the warmth imparted to it by the mother; and the cicatricula or germ-spot (which is the mass of cells first developed from the germ-yolk) is made, by a similar contrivance, always to rise to the highest point.In the eggs of Fishes there is no additional albumen; and in those of Frogs the albumen is common to the general mass of ova, constituting the peculiar "gelatinous envelope," which forms long necklace-like strings of "spawn," within which the black yolk-bags are disposed at tolerably regular intervals. In Mammals, each ovum receives a separate investment of a jellylike substance in its passage along the oviduct into the uterus; and around this there is formed a fibrous membrane termed the Chorion, which is destined to take a very important share in the subsequent nutrition of the embryo (§ 761). 756. In the eggs of Frogs, as in those of Mammals, the whole of the yolk undergoes the process of segmentation already described (§ 736), and takes a share in the formation of the "mulberry mass.' But in the eggs of Fishes, Birds, and the higher Reptiles, this process is limited to that small portion of the yolk which is distinguished as the germ-yolk; and the formation of the germinal membrane takes place after a different fashion. The mass of cells that immediately results from segmentation, flattens itself out on the surface of the yolk, forming the minute semi-opaque whitish spot, which is known as the cicatricula, germ-spot, or "tread;" and by a further extension it constitutes the "germinal membrane," which gradually spreads itself over the food-yolk; at the same time dividing itself into two layers, between which a third is afterwards interposed. Thus the "germinal membrane," which may be compared to the seed-leaves or cotyledons of Plants, forms a sort of temporary stomach round the mass of nutriment prepared for the sustenance of the embryo; the whole of which nutriment, as will be presently seen, is absorbed into the body of the embryo through its instrumentality. 582 PRIMITIVE TRACE: DEVELOPMENT OF VERTEBRAL COLUMN. 757. The first indication of the permanent fabric in all Vertebrated animals, consists of a delicate longitudinal streak, termed the "primitive trace" (fig. 316, b), that is observable in the midst of a pellucid area, which is again surrounded by a Fig. 316.-YOLK-BAG OF FOWL'S EGG, after twelve hours' incubation : a, yolk; b, primitive trace surrounded by pellucid area; c, more opaque ring, the commencement of the vascular area. a ring of more opaque aspect (c). This primitive trace is the foundation of the Vertebral Column. It is in the first instance a mere furrow in the outer layer of the germinal membrane; but the sides of this furrow, known as the dorsal lamina, rise up and arch-over, so as gradually to meet and convert the furrow into a canal. The meeting first takes place in what is afterwards to become the middle of the back; and here we find the first distinct rudiments of the vertebral column, in the condition of a series of small square plates (figs. 317, e, c, 325, 1, 1) on either side, which are the representatives of the arches of as many vertebræ. The furrow widens-out in the situation of the head, so as to form the receptacle (d) for the series of large ganglionic masses that is to constitute the brain (fig. 323, d, e, f); and though its sides do not there close-in for some time longer, it receives a special hood-like covering from a peculiar fold of the germinal membrane, the edge of which is seen at e, fig. 317. The cells, of which the parts of these laminæ that bound the bottom and sides of this furrow are composed, appear to furnish the rudiments of the nervous centres that are afterwards to occupy the canal; but beneath its deepest part there lies a continuous rod of peculiar nucleated cells (f), the chorda dorsalis, which marks-out the situation afterwards to be taken by the bodies of the vertebræ. This remains the only representative of the vertebral column in the Lamprey and other Fishes of a low grade, the development of whose bony skeleton is checked so early that it never advances beyond this simple embryonic type (§ 53). DEVELOPMENT OF CIRCULATING APPARATUS. 583 758. During the progress of this change, another very important one is taking place, which is destined for the nourishment of the embryo during its further development. This is the formation of vessels in the substance of the germinal membrane; which vessels serve to take up the nourishment supplied by the yolk, and to convey it through the tissues of the embryo. The space over which these vessels spread themselves, is called the Vascular Area; it makes its appearance during the second day of incubation in the Fowl's egg (fig. 316, c), and soon spreads itself over the surface of the yolk (figs. 318, 319). Islets or points of a dark colour first appear in it; these unite in rows; and at last continuous vessels are formed. The heart makes its appearance at the twenty-seventh hour of incubation, as a simple dilatation of the trunk into which the blood-vessels unite (fig. 320, h). Its wall is at 584 DEVELOPMENT OF VESSELS AND DIGESTIVE CAVITY. first formed by a layer of cells; and no muscular structure is seen in it, until after its regular pulsations have commenced. It is in these vessels that the first blood is formed; and the same process appears to be continued through the whole period of incubation, the yolk being progressively converted into blood, and this blood being conveyed by the great trunks which collect it into the body of the embryo. Looking at the yolk-bag in the light of a temporary stomach, its vessels may be likened to those which take so large a share in the act of absorption from the digestive cavity of the adult (§ 218). 759. During the same early period of incubation, the layers of the germinal membrane begin to exhibit various folds, which afterwards serve for the formation of the several cavities of the body. The points of it which lie beyond the extremities, and which embryo, are doubled-in so yolk; and their folded edges gradually approach one another under the abdomen, Fig. 319. EMBRYO OF BIRD, WITH THE VESSELS, i, of the vascular area, after four days' incubation. spread-out from the sides of the as to make a depression upon the terior of the egg. In this manner is formed the permanent digestive cavity; which is first a simple pouch communicating with the yolk-bag, by a wide opening, as seen at s, fig. 320; but which is gradually separated from it by the narrowing of this orifice (fig. 322), the connecting portion being elongated into a duct (fig. 321, 6). Thus we may Fig. 320.-DIAGRAM OF THE FORMATION e, embryo; f, g, layers of germinal mem- |