Page:The American Cyclopædia (1879) Volume VI.djvu/572

 564 EMBRYOLOGY day, by a sudden movement it strikes its bill through the end of the attenuated and brittle egg shell, and by inhaling the air and continu- ing its struggles, finally extricates itself from the cavity of the shell, leaving the allantois adherent to its internal surface. The blood vessels of the allantois are torn off at the umbilicus, which afterward closes and unites by a permanent cicatrix. Another important change which takes place in the development of birds and quadrupeds, in addition to those presented by frogs and fishes, is the formation of the urinary apparatus. In fishes and ba- trachians the urinary organs are two long glandular bodies situated on each side the spinal column, which are known as the Wolf- fian bodies, and which remain permanent throughout the life of the animal, no true kid- neys ever being produced. But in birds and quadrupeds the Wolffian bodies, which are at first very large and important organs, disap- pear during the progress of embryonic develop- ment, while the kidneys are formed at the same time, and gradually take their place as urinary organs. The kidneys are accordingly substituted for the Wolffian bodies in these instances, very much as lungs are substituted for gills in the development of the frog. In many species of quadrupeds the allantois at- tains a large size, and performs a very impor- tant function, during intra-uterine life. In the ruminating animals, cows, sheep, goat, deer, &c., it forms an elongated sac, taking the form of the uterine cavity, and lying in close contact with the lining membrane of the uterus. The cavity of this sac communicates with the cavity of the posterior part of the intestine, from which it was originally developed, and receives the secretion of the WolflSan bodies, and after- ward of the kidneys. Its exterior is covered with a large number (60 to 80) of tufted vas- cular prominences, which are entangled with similar elevations of the uterine mucous mem- brane, called cotyledons ; and the blood of the embryo, while circulating through these bodies, absorbs from the maternal vessels the mate- rials requisite for its nutrition. In the pig the allantois is nearly smooth on its external sur- face, nierely presenting transverse folds and ridges, which lie in contact with similar ine- qualities of the uterine mucous membrane. In the carnivorous animals its middle portion is shaggy and vascular, and entangled with the blood vessels of the uterus, while its two ex- tremities are smooth and unattached. In the human embryo the amnion is formed in the same manner as already described ; but the al- lantois, instead of constituting a hollow sac, with a cavity containing fluid and communi- cating with the intestine, spreads out into a continuous flattened membrane, the two lay- ers of which are in contact with each other and adherent, leaving consequently no cavity between them. It extends, however, quite round the foetus, enveloping it in a continuous vascular membrane, which hero takes the name of the chorion. The chorion is, accordingly, the same thing in the human species as the allantois in the lower animals, except that its cavity is obliterated by the adhesion of its walls. It is covered uniformly at an early date with tufted villosities, which become en- tangled with the mucous membrane of the uterus. But during the third month it begins to grow smooth over the greater portion of its surface, while at a certain part the villous tufts grow more rapidly than before, until they are finally converted into a thick vascular, spongy, and velvety mass of villosities, which penetrate into the uterine mucous membrane, and be- come adherent to its blood vessels. This organ is then termed the placenta; and from that time forward it serves the fcetus as an organ of absorption and nourishment, its blood vessels imbibing from the circulation of the mother the albuminous fluids which it requires for growth and nutrition. The amnion in the hu- man species is at an early period so arranged that it closely invests the body of the embryo, while between it and the chorion there is in- terposed a thick layer of soft gelatinous mate- rial. During the second and third months the cavity of the amnion enlarges, by the accumula- tion of a watery and albuminous fluid (the amniotic) in its interior, while the gelatinous matter between it and the chorion is gradually absorbed and disappears, in order to make way for its expansion. By this enlargement the amnion approaches nearer the internal surface of the chorion, and by the beginning of the fifth month the two membranes come in con- tact with each other. By this means the foe- tus becomes enclosed in a large cavity (the amniotic cavity), filled with fluid, so that a free space is allowed for the movements of the foe- tal limbs. These movements begin to be per- ceived about the fifth month, at which time quickening is said to take place. They after- ward become more strongly pronounced, and before birth are frequently very active. These movements are also favored by the formation and growth of the umbilical cord. The blood vessels of the fcetus, termed the umbilical ves- sels, which pass out from the abdomen to the placenta and the chorion, become much elon- gated, and at the same time covered with a de- posit of firm gelatinous matter, the whole be- ing surrounded by a prolongation of the mem- brane of the amnion. This bundle of vessels, covered with the above investments, is termed the umbilical cord. It grows very long, and also becomes spirally twisted upon its own axis, usually in a direction from right to left. There are in the latter periods of gestation two umbilical arteries, carrying the blood of the fcetus outward to the placenta, and one umbilical vein, in which it is returned to the body and the internal venous system. The formation of the blood and blood vessels in the embryo takes place at a very early period. Soon after the production of the blastodermic membrane, some of the cells of which it i