The nourishment of the fœtus in the maternal womb is effected, as is well known, by a peculiar organ, richly supplied with blood at its surface, called the placenta. This important nutritive organ is a spongy, round disk, from six to eight inches in diameter, about an inch thick, and one or two pounds in weight; it is separated after the birth of the child, and issues as the “after-birth.” The placenta consists of two very different parts, the fœtal and the maternal part. The latter contains highly developed sinuses, which retain the blood conveyed to them by the arteries of the mother. On the other hand, the fœtal placenta is formed by innumerable branching tufts or villi, which grow out of the outer surface of the allantois, and derive their blood from the umbilical vessels. The hollow, blood-filled villi of the fœtal placenta protrude into the sinuses of the maternal placenta, and the slender membrane between the two is so attenuated that it offers no impediment to the direct interchange of material through the nutritive blood-stream (by osmosis).
In the older and lower groups of the placentals the entire surface of the chorion is covered with a number of short villi; these “chorion-villi” take the form of pit-like depressions of the mucous membrane of the mother, and are easily detached at birth. That happens in most of the ungulata (the sow, camel, mare, etc.), the cetacea, and the prosimiæ; these “mallo-placentalia” (with a diffuse placenta) have been denominated the indeciduata. The same formation is present in man and the other placentals in the beginning. It is soon modified, however, as the villi on one part of the chorion are withdrawn; while on the other part they grow proportionately stronger, and unite intimately with the mucous membrane of the womb. It is in consequence of this intimate blending that a portion of the uterus is detached at birth, and carried away with loss of blood. This detachable membrane—the decidua—is a characteristic of the higher placentalia, which have, consequently, been grouped under the title of deciduata; to that category belong the carnassia, rodentia, simiæ, and man. In the carnassia and some of the ungulata (the elephant, for instance) the placenta takes the form of a girdle, hence they are known as the zonoplacentalia; in the rodentia, the insectivora (the mole and the hedge-hog), the apes, and man, it takes the form of a disk.
Even ten years ago the majority of embryologists thought that man was distinguished by certain peculiarities in the form of the placenta—namely, by the possession of what is called the decidua reflexa, and by a special formation of the umbilical chord which unites the decidua to the fœtus. It was supposed that the rest of the placentals, including the apes, were without these special embryonic structures. The funiculus umbilicalis is a smooth, cylindrical cord, from sixteen to twenty-three inches long, and as thick as the little finger. It forms the connecting link between the fœtus and the maternal placenta, since it conducts the nutritive vessels from the body of the fœtus to the placenta; it comprises, besides, the pedicle of the allantois and the yelk-sac. The yelk-sac in the human case forms the greater portion of the germinal vesicle during the third week of gestation; but it shrivels up afterwards so that it was formerly entirely missed in the mature fœtus. Yet it remains all the time in a rudimentary condition, and may be detected even after birth as the little umbilical vesicle. Moreover, even the vesicular structure of the allantois disappears at an early stage in the human case; with a deflection of the amnion, it gives rise to the pedicle. We cannot enter here into a discussion of the complicated anatomical and embryological relations of these structures. I have described and illustrated them in my Anthropogeny (twenty-third chapter).
The opponents of evolution still appealed to these “special features” of human embryology, which were supposed to distinguish man from all the other mammals, even so late as ten years ago. But in 1890 Emil Selenka proved that the same features are found in the anthropoid apes, especially in the orang (satyrus), while the lower apes are without them. Thus Huxley’s pithecometra thesis was substantiated once more: “The differences between man and the great apes are not so great as are those between the manlike apes and the lower monkeys.” The supposed “evidences against the near blood-relationship of man and the apes” proved, on a closer examination of the real circumstances, to be strong reasons in favor of it.
Every scientist who penetrates with open eyes into this dark but profoundly interesting labyrinth of our embryonic development, and who is competent to compare it critically with that of the rest of the mammals, will find in it a most important aid towards the elucidation of the descent of our species. For the various stages of our embryonic development, in the character of palingenetic phenomena of heredity, cast a brilliant light on the corresponding stages of our ancestral tree, in accordance with the great law of biogeny. But even the cenogenetic phenomena of adaptation, the formation of the temporary fœtal organs—the characteristic fœtal membranes, and especially the placenta—gives us sufficiently definite indications of our close genetic relationship with the primates.
[CHAPTER V]
THE HISTORY OF OUR SPECIES
Origin of Man—Mythical History of Creation—Moses and Linné—The Creation of Permanent Species—The Catastrophic Theory: Cuvier—Transformism: Goethe—Theory of Descent: Lamarck—Theory of Selection: Darwin—Evolution (Phylogeny)—Ancestral Trees—General Morphology—Natural History of Creation—Systematic Phylogeny—Fundamental Law of Biogeny—Anthropogeny—Descent of Man from the Ape—Pithecoid Theory—The Fossil Pithecanthropus of Dubois
The youngest of the great branches of the living tree of biology is the science we call biological evolution, or phylogeny. It came into existence much later, and under much more difficult circumstances, than its natural sister, embryonic evolution or ontogeny. The object of the latter was to attain a knowledge of the mysterious processes by which the individual organism, plant or animal, developed from the egg. Phylogeny has to answer the much more obscure and difficult question: “What is the origin of the different organic species of plants and animals?”