The works of Francis Maitland Balfour, Volume 3 (of 4)

by Francis M. Balfour

Fig. 161. Diagrammatic representations of the minute structure of the placenta. (From Turner.)
F. the fœtal; M. the maternal placenta; e. epithelium of chorion; e´. epithelium of maternal placenta; d. fœtal blood-vessels; d´. maternal blood-vessels; v. villus.
A. Placenta in its most generalized form.
B. Structure of placenta of a Pig.
C. Structure of placenta of a Cow.
D. Structure of placenta of a Fox.
E. Structure of placenta of a Cat.
F. Structure of placenta of a Sloth. On the right side of the figure the flat maternal epithelial cells are shewn in situ. On the left side they are removed, and the dilated maternal vessel with its blood-corpuscles is exposed.
G. Structure of Human placenta. In addition to the letters already referred to ds, ds. represents the decidua serotina of the placenta; t, t. trabeculæ of serotina passing to the fœtal villi; ca. curling artery; up. utero-placental vein; x. a prolongation of maternal tissue on the exterior of th villus outside the cellular layer e´, which may represent either the endothelium of the maternal blood-vessel or delicate connective tissue belonging to the serotina, or both. The layer e´ represents maternal cells derived from the serotina. The layer of fœtal epithelium cannot be seen on the villi of the fully-formed human placenta.

In the human placenta (G), as in that of Apes, the greatest modification is found in that the maternal vessels have completely lost their capillary form, and have become expanded into large freely communicating sinuses (d´). In these sinuses the fœtal villi hang for the most part freely, though occasionally attached to their walls (t). In the late stages of fœtal life there is only one epithelial layer (e´) between the maternal and fœtal vessels, which closely invests the fœtal villi, but, as shewn by Turner and Ercolani, is part of the uterine tissue. In the fœtal villi the vessels retain their capillary form.

Evolution of the Placenta.

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 fœtus. The fact that in Marsupials both the yolk-sack and the allantois are functional in rendering the chorion vascular makes it à 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 fœtal membranes is actually found. In the primitive Placentalia there was probably present a discoidal allantoic region of the chorion, from which simple fœtal villi, like those of the pig ([fig. 161] B), projected into uterine crypts; but it is not certain how far the umbilical part of the chorion, which was no doubt vascular, may also have been villous. From such a primitive type of fœtal membranes divergences in various directions have given rise to the types of fœtal membranes now existing.

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. (1) by increasing the complexity of the fœtal villi and maternal crypts over a limited area, (2) by increasing the area of the part of the chorion covered by placental villi. Various combinations of the two processes would also of course be advantageous.

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 fœtus.

The arrangement of the fœtal 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 shewn by the fact that the allantoic region of the placenta is at first discoidal (p. [248]). 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 fœtal life in the zonary placenta of the Carnivora is probably due to its being on the whole advantageous to secure the nutrition of the fœtus by insuring a more intimate relation between the fœtal and maternal parts, than by increasing their area of contact. The reason of this is not obvious, but as mentioned below, there are other cases where it can be shewn 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 discoidal placenta is illustrated by the Lemuridæ, the Suidæ, 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 metadiscoidal placenta of Man and Apes, from its mode of ontogeny (p. [248]), is clearly derived from a diffused placenta—very probably similar to that of Lemurs—by a concentration of the fœtal villi, which are originally spread over the whole chorion, to a disc-shaped area, and by an increase in their arborescence.

The polycotyledonary forms of placenta are due to similar concentrations of the fœtal villi of an originally diffused placenta.