Page:The American Cyclopædia (1879) Volume VI.djvu/571

EMBRYOLOGY 563 I perfect insect, or imago, capable of rapid and sustained flight, ornamented with brilliant col- ors, provided with different sensory and diges- tive organs and a well developed sexual appa- ratus. In those instances where the hatch- ing of the egg is a longer process, similar changes to the above take place while the em- bryo is still retained in its interior. At the same time certain other organs are formed in addition, which either disappear before the time of hatching, or are thrown off when the young animal leaves the egg. With turtles, for example, the eggs, consisting of the vitel- lus, albumen, and shell, are deposited in an excavation in the earth or sand, and allowed to hatch in these situations. With birds, they are placed usually in nests, formed of twigs, leaves, and fibres, and there kept constantly warmed and protected by contact with the body of the female parent. This process is termed incubation, and may be imitated arti- ficially by keeping the eggs at a temperature of 104 F. and providing for the regular supply of fresh air and a proper regulation of the at- mospheric moisture. During incubation the eggs of the common fowl lose 12 per cent, of their weight, of which 11 per cent, is due to the exhalation of moisture. They also absorb oxygen and exhale carbonic acid. The segmen- tation of the vitellus and formation of the blas- todermic membrane, and of the organs of the embryo, take place for the most part according to the plan already described, but variations present themselves which make the process more complicated. The vitellus, for example, instead of being entirely surrounded by the abdominal walls, is divided into two portions by a constriction situated about its middle. One of these portions remains outside the abdomen of the embryo, though still connected with it by a narrow neck, and by blood vessels which ramify upon its surface. This sac, containing a portion of the vitellus, is called the umbilical vesicle. It supplies the embryo with nourish- ment during the whole period of incubation ; immediately after the egg is laid the albu- en, which is at first gelatinous in consistency, begins to liquefy near the upper surface, and the liquefied portions are immediately absorbed into the yolk. The yolk, therefore, grows larger and more fluid than before, while the albumen diminishes in quantity, and loses its watery portions. The blood vessels of the em- bryo, ramifying over the surface of the vitel- lus and the umbilical vesicle, in their turn ab- sorb the nutritious fluids from it, and convey them into the interior of the body, to be used in the formation of the tissues. At the end of incubation the albumen has disappeared and the umbilical vesicle has much diminished in size, while the body of the chick has increased at the expense of both ; but the umbilical ves- icle, containing the remains of the yolk, still exists, and is enclosed within the abdominal walls when the chick leaves the egg. In quadrupeds and the human species the umbili- cal vesicle is much smaller in proportion to the body, and less important in function, than in birds and the scaly reptiles. In the human embryo the umbilical vesicle, always very small, diminishes rapidly soon after the end of the third month, and is hardly distinguishable during the latter period of gestation. In the egg of the fowl certain accessory membranes or envelopes begin to grow around the embryo at an early period. The first of these is the amnion, which is formed by a double fold of the blastodermic membrane, rising up about the edges of the body of the embryo, so as to surround it by a kind of circumvallation, or embankment. By continued growth these folds at last approach each other and meet over the back of the embryo, forming by their union and adhesion an enclosing membrane, or sac, which is the amnion. The amnion, there- fore, is a membranous envelope, which is closed over the back of the embryo, but which re- mains open in front of the abdomen. About the same time a vascular, membranous diver- ticulum grows out from the alimentary canal, near its posterior extremity, and emerging from the open part of the abdomen turns upward over the back of the embryo, outside the am- nion, and just inside the shell membranes. This vascular membrane is the allantois, an organ which surrounds the embryo in quadru- peds, birds, and some reptiles, and which serves as an organ of nourishment, or for the aera- tion of the blood. It first makes its appear- ance in the early stages of embryonic life, as we have mentioned, in the form of a protru- sion or offshoot from the lower part of the ali- mentary canal. It rapidly increases in extent, protrudes further from the abdomen, spreads laterally in every direction, and thus finally envelopes the body of the foetus ; its edges, as they come in contact with each other from op- posite directions, in many species becoming adherent, so as to form a continuous mem- brane. This membrane, which in the bird's egg is situated immediately underneath the shell, is abundantly supplied with blood ves- sels coming from the interior of the abdomen of the embryo, and returning thither after having ramified upon the surface of the allan- tois. It is by this means that the absorption of oxygen and exhalation of carbonic acid take place, which are so distinctly marked in the fowl's egg during the latter period of incu- bation. The allantois in these animals is ac- cordingly an organ of respiration for the foetus, Toward the latter period of incubation the al- lantois becomes very closely adherent to the egg shell, and the shell itself grows thinner, more porous, and more fragile; whence it is believed that the allantois also serves to ab- sorb calcareous matter from the shell, which it conveys into the interior of the body, to be used in the formation of the bones, the ossifi- cation of which takes place about this period. When the chick is sufficiently developed to leave the egg, usually at the end of the 21st