Page:The American Cyclopædia (1879) Volume VI.djvu/573

 EMBRYOLOGY 565 composed break down, or separate from each other in such a manner as to leave irregular spaces, or canals, which inosculate with each other by frequent communications. These canals are destined afterward to become the blood vessels, the structure of which is gradu- ally perfected by the growth of fibrous tissue in their walls, and their complete separation from the neighboring parts. In the interior of these canals, or imperfectly formed blood vessels, there is to be seen at first only a transparent, colorless fluid, holding in suspen- sion a few large, roundish, nucleated cells, which move sluggishly to and fro, as the cur- rent of the circulating fluid begins to be estab- lished. These cells do not differ much at this period from those which constitute the general mass of the neighboring tissues; but soon af- terward they begin to be modified in their ap- pearance, and give place to true blood globules, Their surface becomes smooth, and a reddish coloring matter is produced in their interior, which gives them a tinge similar to that of the red globules of the blood in the adult condi- tion. The red blood globules of the foetus, however, still differ in several important par- ticulars from those of the adult. They are considerably larger and more globular in shape, and have also a very distinct nucleus, which is wanting in the blood globules of the adult, at least in the quadrupeds. They increase in numbers also at this time by spon- taneous division, one globule becoming divi- ded into two, which separate from each other and afterward become themselves divided in a similar manner. In this way the quantity of the blood globules is very rapidly increased, and they soon become also still further altered in form and structure. They diminish in size, be- come in the human subject and the quadruped flattened and biconcave in form, and finally the nucleus disappears. These changes are all ef- fected during foetal life, and chiefly during the early months, so that at the time of birth the blood globules have already the characteristics which distinguish them in adult life. The multi- plication of the blood globules by subdivision is a process which takes place only in the embryo. The perfectly formed blood globules increase in number in some other way, probably by the isolated production and growth of new cells. At the time of birth the fcetal membranes (amnion and chorion) are ruptured, and the foetus escapes. The umbilical cord being at the same time divided and tied, the portion still con- nected with the foetus soon shrivels and sepa- rates by spontaneous ulceration, while the spot at which it was attached heals in a few days, leaving a cicatrix on the middle of the abdo- men, which is permanent throughout life, and which _is called the umbilicus. The limbs grow, by a kind of budding or sprouting process, from the side of the body. They are at first mere rounded eminences, without distinction of parts or articulations ; but they subsequently become successively divided into fingers and toes, and the different joints of the arm and leg. The upper extremities during the greater part of foetal life are larger than the lower, but after- ward the lower extremities and the pelvis grow faster than the arms and shoulders, and finally become after birth much the larger of the two. The lungs are small and solid in texture before birth, but immediately afterward they expand by the inhalation of air, and receive a much larger supply of blood than before. On the other hand, the liver is much larger in propor- tion to the rest of the body at an early period than subsequently. In some animals it amounts during the first part of foetal life to 12 per cent, of the entire weight of the body, and is reduced to 3 or 4 per cent, at the time of birth. In the human subject it is equal at birth to 3 per cent, of the entire weight, but is reduced in the adult to less than 3 per cent. Great changes take place also during foetal life in the anatomy of the heart and circulatory system, as well as is the relative size and development of nearly all the organs in the body. These changes continue to take place after birth, though less rapidly than before, and the entire process of develop- ment is not regarded as complete until the indi- vidual has reached the adult condition. A very singular modification of the above process of embryonic development among the mammalia occurs in the marsupial animals, of which the American opossum (didelphys Virginiana) is a representative. In these animals the eggs are impregnated and the formation of the embryo commenced in the usual way ; but after remain- ing for a comparatively short time in the uterus, and while their development is still very incom- plete, the embryos are discharged from the gen- erative passages, and are immediately afterward found attached by the mouth to the teats of the parent. They are then less than half an inch in length, and quite gelatinous and embryonic in appearance. They are protected by a double fold of the integument of the abdomen, which forms a kind of pouch, surrounding the teats, and serving to enclose the young and helpless embryos. They remain in this situation during the completion of their development, continuing attached for the most part to the teats, from which they derive nourishment ; and even after they have become capable of running about by themselves, they still, upon an alarm, take refuge for a time in the pouch as before. It is not known how the young embryos, when expelled from the uterus, find their way into the external pouch so as to reach the teats, for, notwith- standing many attempts have been made to as- certain this point, the animal is so secret in her habits at the time of delivery that they have been thus far entirely unsuccessful. Among in- vertebrate animals the egg is constituted, as a general thing, in nearly the same way as inver- tebrata, and its impregnation takes place also in a similar manner. The segmentation of the y ol k goes on by repeated subdivisions, until the whole vitellus is converted into a mulberry-shaped mass out of which the embryo is formed.