Part of the allantois becomes the urinary bladder, part of it forms the umbilical cord, and part of it enters into the formation of the placenta by its union with the chorion.
The chorion is the outermost layer of the foetal envelope and is of later development; it is formed by the allantois fusing with the external layer of the amnion, and these in turn become amalgamated with the vitelline membrane to form a new membrane, which receives the name of "chorion".
The chorion then becomes covered externally with a multiplicity of little vascular tufts which give it a shaggy appearance. These little tufts, called chorionic villi, consist essentially of minute arterioles and veinlets, held together by connective tissues. These villi are concerned in the early nutrition of the fcetus. They eventually disappear from two thirds of the surface of the chorion, leaving this part smooth, and the remaining one third remains shaggy with the vascular tufts, and forms the foetal part of the placenta.
Just here it will be well to remind the reader that the term ovum is used in various senses. In the preceding pages we applied it to the female reproductive element, or immature ovum; to the impregnated ovum or oosperm; and to the various later stages of development. We started by calling it a single cell, and later on described it as consisting of myriads of cells. This seems strange indeed, but the reason is to be found in the fact that all the developmental changes of the embryo and of the foetal membranes, take place entirely within the original cell-wall, or vitelline membrane, which of course becomes enormously distended by the wonderful changes which occur within it.
With this use of the term ovum, a woman who has arrived at the end of the pregnancy, and who has a full time child within her uterus, is yet spoken of as carrying an entire ovum, the fcetus, of course, constituting by far the most important part of the ovum.
Furthermore we must acknowledge that the foregoing changes have never all been observed in the human embryo, but that we derive our information from a study of comparative embryology, concluding by a justifiable inference that the processes exactly describe the early conditions found in man. "The embryos of a man, dog, seal, bat, reptile, etc., can at first hardly be distinguished from each other".
Among animals the exact date of coitus can be readily fixed, and at any subsequent day the female can be killed for the purpose of studying the development of the embryo. Obviously it is impossible to observe these changes in a woman except by an occasional accident.
No human ovum has ever been seen and described during the first week of embryonic development, and very few as early as the third week.
Reichert's ovum is one of the earliest ones to have been described. It was taken from the womb of a woman who committed suicide, as supposed, thirteen days after impregnation.
The placenta, or "after-birth," is the organ of circulation, nutrition, excretion and respiration of the fcetus, and the structure by which the foetus is attached to the wall of the uterus by means of the umbilical cord, or "navel-string."