We observe here at once that the majority of animals having a discoidal placenta subsist chiefly upon fruits and grains, and that the typical representatives of this class, namely, those whose plactental formation is most distinctly discoidal, are also the most exclusively frugivorous.
Here, as elsewhere in nature, an exact line cannot be drawn. Transitional forms exist everywhere, and to this the placenta is no exception. The most striking accordance, however, exists between the placenta of man and that of the tailless apes—namely, the gorilla, orang, chimpanzee and gibbon. Between other discoidal species, the differentiation, though minute, is clearly marked; but between man and these apes the resemblance is so exact as to stamp them plainly as members of the same family.
The completely developed placenta is in the form of a circular disc, about eight inches broad, one inch thick and weighing about two pounds. Its manner of development is identical in the human subject and that of the above-named anthropoid apes. Its exact formation is thus described by Huxley:
“From the commencement of gestation, the superficial substance of the mucous membrane of the human uterus undergoes a rapid growth and textural modification, becoming converted into the so-called decidua. While the ovum is yet small, this decidua is departable into three portions: The _decidua vera_, which lines the general cavity of the uterus; the _decidua reflexa_, which immediately invests the ovum; and the _decidua serotina_, a layer of especial thickness, developed in contiguity with those chorionic villi which persist and become converted into the fœtal placenta. The _decidua reflexa_ may be regarded as an outgrowth of the _decidua vera_ the _decidua serotina_ as a special development of a part of the _decidua vera_. At first, the villi of the chorion are loosely implanted into corresponding impressions of the _decidua_; but, eventually, the chorionic part of the placenta becomes closely united with and bound to the uterine _decidua_, so that the fœtal and maternal structures form one inseparable mass.”
The fœtus thus united to the mother is nourished by means of numerous arterial and venous trunks, which traverse the deeper substance of the uterine mucous membrane, in the region of the placenta. These are connected with the placenta by means of the umbilical cord, which consists of two arteries and two veins. The length of this cord is greater in the case of man and the anthropoid apes than in any other animals, reaching in them a length of about two feet. The strict accordance which thus appears between the placental structure of man and the ape indicates, upon the basis of Huxley’s principles of classification, the same physiological functions _and the same dietetic character_. There exists a complete similarity between the corresponding organs in each: Their extremities end in hands and feet. Their teeth and digestive apparatus indicates a frugivorous diet. Their breasts and manner of nursing suggest the same tender care of the new-born creature; while the brain and mental capacity are also of a like character—differing only in degree; indeed, the difference between the ape and animals of the next lower grade is much greater than between the ape and man; there being in the latter case really no essential anatomical or physiological differences.