A further point of interest is the anatomical relations of the ovum, in dependent of the foetus itself. Three conditions may obtain, and they seem to correspond with the three grand etiological divisions already re ferred to: 1. Two ova from the same ovary, but belonging to different Graafian follicles.—The two ovules are independently developed, each one having its own membranes, and, consequently, two chorions and two separate amnions. Each foetus is thus contained in a separate compartment. At first, each ovum possesses a decidna reflexa, so that when the originally separate ovules become attached to each other as a result of their develop ment, the line of contact represents a septum with six layers, two decidna3 reflexte, two chorious, and two amnions. But the decidua3 reflexte having been reabsorbed, there finally remain only two amnions, two chorions and a single decidua vent. Thus, each foetus has a separate and independent placenta, but the two placentas may form a single mass plainly divided into two parts by a membranous partition. The two circulations are quite independent, and each foetus has a separate funis. In other cases this septum is wanting, and the two placentas seem to form a single mass possessing two umbilical cords. But, even here, the union of the placentas is only apparent, for the septum still presents the four layers formed by the two amnions and the two chorions. The circulations are still inde pendent.
2. A single Graafian follicle, and two ovules simultaneously fecundated. —There are two amnions but only a single chorion The two ova orig inally came into close relations with each other in ntero. There was, at first, only one reflexa, but still two chorions and two amnions, but these chorions became so intimately associated that they finally only formed a single chorion, and the septum was formed of two layers, the two amnions. The two chorions, which at first existed in the septum, have been reab sorbed. In this case the placentas are still united into a single mass, but the independence of the two circulations is not so absolute, and anasto moses between the branches of the two circulations are not rare. Accord ing to Jacquemier, these anastomoses are almost entirely venous and superficial, embracing only large venous branches. The independence of the fcetuses, although not so complete as in the former case, is still almost perfect, and hence the possibility of the death of one foetus and of the survival of the other. Each foetus here possesses a distinct fnnis.
3. One ovule with two nuclei.—There is, in this case, a single chorion and a single amnion. After fecundation, two embryonic spots appear upon the same blastoderm, and the chorion is single from the start. Not so with the amnions, which are originally double, but which, their septum disappearing, become a single sac formed by a single chorion and a single amnion containing two foetuses. The placenta, in this case, always forms a single mass. The two foetal circulations intercommunicate through superficial and deep anastomoses, both venous and arterial. Never theless, this communication between the two circulations, although the rule, is not always present. Sometimes two separate cords connect the
placenta and the fcetuses, and sometimes, although more rarely, a single cord, proceeding from the placenta, soon divides into two branches, one for either fwtus. We find in the thesis of Chantreuil two curious cases, one of William Newman, the other of Soete. In the former, the cord of the first child was tied into a single knot, and surrounded a coil in the second cord, so that the latter was completely strangulated. In Soete's case the two cords, which were both very long, together formed, at about their middle, a perfect double knot. We refer to the article, " Labor in Twin Pregnancy;' for remarks about the presentations and positions.
Although the combined volume of the two fcetuses generally exceeds the volume and weight of a single mature fcetus, each is, separately considered, below the average, even when labor occurs at full term. However, it is not unusual to see twins quite as fully developed as single children. In two cases of twin pregnancy, seen by me in private practice, the children weighed as follows: In one case the combined weight of the two children, which were males, was a little over thirteen pounds. The first child weighed about seven and the second about six and one half pounds. In the second case the two children weighed, together, fourteen and one-half pounds. The first child, the boy, much the larger, weighed nine pounds. The second, the girl, weighed five and one-half pounds. Difference, three and one-half pounds. In the first case the mother was a primipara, in the second a multipara. These cases are ex ceptional, but what is still rarer is the difference in the weight of the children. Generally one of them is much more fully developed than the other. Sometimes one child dies during the early months of intrauterine life, while the other survives. This is explained by the independence of the two circulations. The dead child remains in utero and is expelled with its twin, but presents an appearance peculiar to itself. As the result of the absorption of its amniotic fluid, it undergoes a peculiar change, called mummification. Being compressed by its fellow, it grows flat, and becomes, aside from its color, which remains white, quite like certain well-known little ginger-bread images. At birth, it is found closely applied to the placenta, enclosed in what was once its foetal sac. It is not very rare, in twin pregnancies, to see one foetus well-formed, and the other presenting deformities or monstrosities. Cazeaux and Tarnier cite examples. These cases occur especially in premature deliveries and particularly in hydramnions. At times, the twins adhere to each other by means of different members, constituting the varieties designated by Geoffroy Saint-Hilaire by the names cephalopages, sternopages, xiphopages, ischiopages, pygopages, etc. In still other cases one germ being included within the other, one foetus absorbs the other, which it partly surrounds. These are cases of foetal inclusion. Finally, certain coccygeal foetal tumors seem to be due to abortive twin pregnancies. We shall refer to these, in detail, in the chapter on " Monstrosities."