The Works of Francis Balfour 1-21
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Foster M. and Sedgwick A. The Works of Francis Balfour Vol. I. Separate Memoirs (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. II. A Treatise on Comparative Embryology 1. (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. III. A Treatise on Comparative Embryology 2 (1885) MacMillan and Co., London.
Foster M. and Sedgwick A. The Works of Francis Balfour Vol. IV. Plates (1885) MacMillan and Co., London.
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Vol I. Separate Memoirs (1885)
XXI. On the evolution of the Placenta, and on the possibility of employing the characters of the Placenta in the classification of the Mammalia
FROM Owen's observations on the Marsupials it is clear that the yolk-sack in this group plays an important (if not the most important) part, in absorbing the maternal nutriment destined for the foetus. The fact that in Marsupials both the yolk-sack and the allantois are concerned in rendering the chorion vascular, makes it a priori probable that this was also the case in the primitive types of the Placentalia ; and this deduction is supported by the fact that in the Rodentia, Insectivora, and Cheiroptera this peculiarity of the foetal membranes is actually found. In the primitive Placentalia it is also probable that from the discoidal allantoic region of the chorion simple foetal villi, like those of the Pig, projected into uterine crypts ; but it is not certain how far the umbilical region of the chorion, which was no doubt vascular, may also have been villous. From such a primitive type of fcetal membranes divergencies in various directions have given rise to the types of foetal membranes found at the present day.
In a general way it may be laid down that variations in any direction which tended to increase the absorbing capacities of the chorion would be advantageous. There are two obvious ways in which this might be done, viz. (i) by increasing the complexity of the foetal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by the placental villi. Various combinations of the two processes would also, of course, be advantageous.
1 From the Proceedings of the Zoological Society of London, t88i.
THE EVOLUTION OF THE PLACENTA. 735
The most fundamental change which has taken place in all the existing Placentalia is the exclusion of the umbilical vesicle from any important function in the nutrition of the foetus.
The arrangement of the foetal parts in the Rodentia, Insectivora, and Cheiroptera may be directly derived from the primitive form by supposing the villi of the discoidal placental area to have become more complex, so as to form a deciduate discoidal placenta, while the yolk-sack still plays a part, though physiologically an unimportant part, in rendering the chorion vascular.
In the Carnivora, again, we have to start from the discoidal placenta, as evinced by the fact that in the growth of the placenta the allantoic region of the placenta is at first discoidal, and only becomes zonary at a later stage. A zonary deciduate placenta indicates an increase both in area and in complexity. The relative diminution of the breadth of the placental zone in late foetal life in the zonary placenta of the Carnivora is probably due to its being on the whole advantageous to secure the nutrition of the foetus by insuring a more intimate relation between the foetal and maternal parts, than by increasing their area of contact. The reason of this is not obvious, but, as shewn below, there are other cases where it is clear that a diminution in the area of the placenta has taken place, accompanied by an increase in the complexity of its villi.
The second type of differentiation from the primitive form of placenta is illustrated by the Lemuridae, the Suidae, and Manis. In all these cases the area of the placental villi appears to have increased so as to cover nearly the whole subzonal membrane, without the villi increasing to any great extent in complexity. From the diffused placenta covering the whole surface of the chorion, differentiations appear to have taken place in various directions. The placenta of Man and Apes, from its mode of ontogeny, is clearly derived from a diffused placenta (very probably similar to that of Lemurs) by a concentration of the foetal villi, which are originally spread over the whole chorion, to a disk-shaped area, and by an increase in their arborescence. Thus the discoidal placenta of Man has no connexion with, and ought not to be placed in, the same class as those of the Rodentia, Cheiroptera, and Insectivora.
736 THE EVOLUTION OF THE PLACENTA.
The polycotyledonary forms of placenta are due to similar .concentrations of the fcetal villi of an originally diffused placenta.
In the Edentata we have a group with very varying types of placenta. Very probably these may all be differentiations within the group itself from a diffused placenta such as that found in Manis. The zonary placenta of Orycteropus is capable of being easily derived from that of Manis by the disappearance of the fcetal villi at the two poles of the ovum. The small size of the umbilical vesicle in Orycteropus indicates that its discoidal placenta is not, like that of the Carnivora, directly derived from a type with both allantoic and umbilical vascularization of the chorion. The discoidal and dome-shaped placentae of the Armadillos, Myrmecophaga, and the Sloths may easily have been .formed from a diffused placenta, just as the discoidal placenta of the Simiidse and Hominidae appears to have been formed from a diffused placenta like that of the Lemuridae.
The presence of zonary placentae in Hyrax and ElepJias does not necessarily afford any proof of affinity of these types with the Carnivora. A zonary placenta may be quite as easily derived from a diffused placenta as from a discoidal placenta ; and the presence of two villous patches at the poles of the chorion in
- Elephas very probably indicates that its placenta has been evolved
from a diffused placenta.
Although it would not be wise to attempt to found a classification upon the placental characters alone, it may be worth while to make a few suggestions as to the affinities of the orders of Mammalia indicated by the structure of the placenta. We clearly, of course, have to start with forms which could not be grouped with any of the existing orders, but which might be called the Protoplacentalia. They probably had the primitive type of placenta described above : the nearest living representatives of the group are the Rodentia, Insectivora/and Cheiroptera. Before, however, these three groups had become dis.tinctly differentiated, there must have branched off from the .primitive stock the ancestors of the Lemuridae, the Ungulata, and the Edentata.
It is obvious on general anatomical grounds that the Monkeys and Man are to be derived from a primitive Lemurian type ; and
THE EVOLUTION OF THE PLACENTA. 737
with this conclusion the form of the placenta completely tallies. The primitive Edentata and Ungulata had no doubt a diffused placenta which was probably not very different from that of the primitive Lemurs ; but how far these groups arose quite independently from the primitive stock, or whether they may have had a nearer common ancestor, cannot be decided from the structure of the placenta. The Carnivora were certainly an offshoot from the primitive placental type which was quite independent of the three groups just mentioned ; but the character of the placenta of the Carnivora does not indicate at what stage in the evolution of the placental Mammalia a primitive type of Carnivora was first differentiated.
No important light is thrown by the placenta on the affinities of the Proboscidea, the Cetacea, or the Sirenia ; but the character of the placenta in the latter group favours the view of their being related to the Ungulata.