the foetus and the vitelline sac, and taking the place of the albumen which has been liquefied and absorbed. It will also be seen, by reference to the figure, that the umbilical vesicle is at the same time formed by the separation of part of the vitellus from the abdomen of the chick; and the vessels of the area vasculosa, which were at first distributed over the vitellus, now ramify, of course, upon the surface of the umbilical vesicle. At last the allantois, by its continued growth, envelopes nearly the whole of the remaining contents of the egg; so that toward the later periods of incubation, at whatever point we break open the egg, we find the internal surface of the shell-membrane everywhere lined with a vascular membranous expansion, supplied by arteries which emerge from the abdomen of the foetus. It is easy to see, accordingly, with what readiness the absorption and exhalation of gases may take place by means of the allantois. The air penetrates from the exterior through the minute pores of the calcareous shell, and then acts upon the blood in the vessels of the allantois very much in the same manner that the air in the minute bronchial tubes and air-vesicles of the lungs acts upon the blood in the pulmonary capillaries. Examination of the egg, furthermore, at various periods of incubation, shows that changes take place in it which are entirely analogous to the process of respiration. The egg, in the first place, during its development, loses water by exhalation. This exhalation is not a simple effect of evaporation, but is the result of the nutritive changes going on in the interior of the egg; since it does not take place, except in a comparatively slight degree, in unimpregnated eggs, or in those which are not incubated, though they may be freely exposed to the air. The exhalation of fluid is also essential to the processes of development, for it has often been found, in hatching eggs by artificial warmth, that if the air of the chamber in which they are inclosed become unduly charged with moisture, so as to retard or prevent further exhalation, the eggs readily become spoiled, and the development of the embryo is arrested. The loss of weight during natural incubation, principally due to the exhalation of water, has been found by Baudrimont and St. Ange' to be over 15 per cent. of the entire weight of the egg. Secondly, the egg absorbs oxygen and exhales carbonic acid. The two observers mentioned above, ascertained that during eigh ' Du Développement du Fœtus. Paris, 1850, p. 143. teen days' incubation, the egg absorbed nearly 2 per cent. of its weight of oxygen, while the quantity of carbonic acid exhaled from the sixteenth to the nineteenth day of incubation amounted to no less than 3 grains in the twenty-four hours.' It is curious to observe, also, that in the egg during incubation, as well as in the adult animal, more oxygen is absorbed than is returned by exhalation under the form of carbonic acid, It is evident, therefore, that a true respiration takes place by means of the allantois, through the membranes of the shell. The allantois, however, is not simply an organ of respiration; it takes part also in the absorption of nutritious matter. As the process of development advances, the skeleton of the young chick, at first entirely cartilaginous, begins to ossify. The calcareous matter, necessary for this ossification, is, in all probability, derived from the shell. The shell is certainly lighter and more fragile toward the end of incubation than at first; and, at the same time, the calcareous ingredients of the bones increase in quantity. The limesalts, requisite for the process of ossification, are apparently absorbed from the shell by the vessels of the allantois, and by them transferred to the skeleton of the growing chick; so that, in the same proportion that the former becomes weaker, the latter grows stronger. This diminution in density of the shell is connected not only with the development of the skeleton, but also with the final escape of the chick from the egg. This deliverance is accomplished mostly by the movements of the chick itself, which become, at a certain period, sufficiently vigorous to break out an opening in the attenuated and weakened egg-shell. The first fracture is generally accomplished by a stroke from the end of the bill; and it is precisely at this point that the solidification of the skeleton is most advanced. The egg-shell itself, therefore, which at first only serves for the protection of the imperfectly-formed embryo, afterward furnishes the materials which are used to accomplish its own demolition, and at the same time to effect the escape of the fully developed foetus. Toward the latter periods of incubation, the allantois becomes more and more adherent to the internal surface of the shell-membrane. At last, when the chick, arrived at the full period of development, escapes from its confinement, the allantoic vessels are torn off at the umbilicus; and the allantois itself, cast off as a use Op. cit., pp. 138 and 149. less and effete organ, is left behind in the cavity of the abandoned egg-shell. The allantois is, therefore, strictly speaking, a foetal organ. Developed as an accessory structure from a portion of the intestinal canal, it is exceedingly active and important during the middle and latter periods of incubation; but when the chick is completely formed, and has become capable of carrying on an independent existence, both the amnion and the allantois are detached and thrown off as obsolete structures, their place being afterward supplied by other organs belonging to the adult condition. CHAPTER X. DEVELOPMENT OF THE EGG IN THE HUMAN WE have already described, in a preceding chapter, the manner in which the outer lamina of the amniotic fold becomes adherent to the adjacent surface of the vitelline membrane, so as to form with it but a single layer; and in which these two membranes, thus fused and united with each other, form at that time the single external investing membrane of the egg. The allantois, in its turn, afterward comes in contact with the investing membrane, and lies immediately beneath it, as a double vascular membranous sac. In the egg of the human subject the development of the membranes, though carried on essentially upon the same plan with that which we have already described, undergoes, in addition, some further modifications, which we shall now proceed to explain. The first of these peculiarities is that the allantois, after spread Fig. 196. At ing out upon the inner surface of HUMAN OVUM, about the end of the first ed into a simple vascular membrane. month; showing formation of chorion. - 1. Umbilical vesicle. 2. Amnion. 3. Chorion. (Fig. 196.) This membrane, moreover, being, after a time, thoroughly fused and united with the two which have preceded it, takes the place which was previously occupied by them. It is then termed the chorion, and thus becomes the sole external investing membrane of the egg. We find, therefore, that the chorion, that is, the external coat or investment of the egg, is formed successively by three distinct membranes, as follows: first, the original vitelline membrane; secondly, the outer lamina of the amniotic fold; and, thirdly, the allantois; the last predominating over the two former by the rapidity of its growth, and absorbing them into its substance, so that they become finally completely incorporated with its texture. It is easy to see, also, how, in consequence of the above process, the body of the foetus, in the human egg, becomes inclosed in two distinct membranes, viz., the amnion, which is internal and continuous with the foetal integument, and the chorion, which is external and supplied with vessels from the cavity of the abdomen. The umbilical vesicle is, of course, situated between the two; and the rest of the space between the chorion and the amnion is occupied by a semi-fluid gelatinous material, somewhat similar in appearance to that of the vitreous body of the eye. The obliteration of the cavity of the allantois takes place very early in the human subject, and, in fact, keeps pace almost entirely with the progress of its growth; so that this organ never presents, in the human egg, the appearance of a hollow sac, filled with fluid, but rather that of a flattened vascular membrane, enveloping the body of the foetus, and forming the external membrane of the egg. Notwithstanding this difference, however, the chorion of the human subject is, in respect to its mode of formation, the same thing with the allantois of the lower animals; its chief peculiarity consisting in the fact that its opposite surfaces are adherent to each other, instead of remaining separate and inclosing a cavity filled. with fluid. The next peculiarity of the human chorion is, that it becomes shaggy. Even while the egg is still very small, and has but recently found its way into the uterine cavity, its exterior is already seen to be covered with little transparent prominences, like so many villi (Fig. 196), which increase the extent of its surface, and assist in the absorption of fluids from without. The villi are at this time quite simple in form, and altogether homogeneous in structure. As the egg increases in size, the villi rapidly elongate, and become divided and ramified by the repeated budding and sprouting of lateral offshoots from every part. After this process of growth has gone on for some time, the external surface of the chorion presents |