Placenta with persistent Umbilical Vesicle (a), and Omphalo-mesenteric Vessels (b, b, b, b). (Reduced by about one-third.) b b that it is a very fair working hypothesis to conclude that the nondevelopment of the amnion which brings pressure to bear upon the posterior end of the embryo, must necessarily influence also the allantoic projection (or 'Bauchstiel') which springs therefrom. The pressure may even be so great as to prevent the formation of the posterior part of the bowel from which the allantois springs, and if both these parts have their development arrested it is easy to understand the absence of the bladder and urachus. On the other hand, the umbilical vesicle will be outside the pressure sphere or may be formed before the amnion begins to exert its compressing force and so remains intact. Its vessels also, at least one pair, will escape, and there seems no room for any explanation other than that they must pass to the chorion and vascularise its villi in the future placental site. The objection that the omphalo-mesenteric vessels may not be able to persist till the full term may be fully met by the specimen of which I now show a drawing. It represents a placenta and cord from an outwardly normal infant born in the Edinburgh Maternity Hospital some years ago, and it shows persistent omphalomesenteric vessels (Plate iii., b b b b) in the cord, which can be easily traced to the remnant of the umbilical vesicle (a) on the foetal surface of the after-birth. The allantoidal vessels were also present in this case. I do not affirm that in this case the chorion was vascularised by the vitelline vessels, although I think it may well have been so to begin with, but I do hold that it proves that they may be found in a functional state at the full term, and that they may grow in length pari passu with the umbilical cord. It seems, therefore, that if the current theory of the teratogenesis of sympodia be accepted, it must be reckoned as forming yet another argument in favour of the vitelline origin of the foetal placenta in such monstrosities. The occurrence of a vitelline placenta in sympodial foetuses has not, however, remained unsuspected up till now, although I may say that I was unaware of this fact at the time when I examined my specimen and formed my views regarding it. In 1880 Colomiatti (118), in his Frammenti di embriologia patologica, a work little known in this country, as a result of the study of various monstrosities (none of which was a sympodial fœtus), came to the conclusion that when one artery only was found in the umbilical cord it might be truly umbilical (i.e., allantoidal), but it might also be omphalo-mesenteric. In the latter case there is usually absence of the bladder, urethra, and lower part of the intestine, and the vascular part of the foetal placenta is not formed from the allantois, but from the umbilical vesicle ('e la parte vascolare della placenta fetale non è fatta dall' allantoide ma della vescicola ombelicale'). Colomiatti also applied this conclusion to the placenta in a case of sympodia which had been reported by Calori. In 1886 Weigert (119), evidently in ignorance of Colomiatti's paper, drew somewhat similar conclusions from the study of cases of single 'umbilical' artery. He described the origin of the vessel from the aorta, and regarded it as a persistent omphalo-mesenteric artery; but he does not seem to have grasped the full import of his observation, for he simply stated that this vessel therefore carried the blood to the cord and placenta. The new light which might thus be thrown upon the development of the placenta seems not to have been evident to Weigert. Labougle and Régnier (92), however, grasped the situation much more fully. In their communication, published in 1889, they very completely described a sympodial monstrosity, and came to the conclusion that there was in it total absence of the allantois and its derivatives. How, then, they asked themselves, was the placenta formed? After a reference to the vitelline placenta of the marsupials, as demonstrated by Owen, Balfour, and Osborn, Labougle and Régnier hazarded the hypothesis that possibly in their sympodial foetus the umbilical vesicle may have taken on the functions of the non-existent allantois. They were careful, however, to emphasise the hypothetical nature of the suggestion, and to confess that absolute proof was not forthcoming. Finally, Arnold (110), in 1894, alluded to the views of Weigert and Labougle and Régnier, but without expressing a personal opinion thereupon. 3. The Occurrence of a Vitelline Placenta in other Terata than the Sympodial. To my third question (Are there any other forms of monstrosity in which a vitelline placenta seems to occur?) I cannot here give a full answer. That I must reserve for a separate communication. I may, however, indicate that I hope to show that in many forms of terata (and even in foetuses not externally monstrous) in which the umbilical cord contains an anomalous number of vessels, and in which the bladder and the lower end of the intestine are absent, there is good reason to believe that the vascular part of the foetal placenta owes its origin to the vitelline and not to the allantoic circulation. In exomphalos, also, where often no cord is formed, and the vessels pass directly from the abdomen to the placenta, the latter in all probability is vitelline. In the well-known placental parasites (acardiac, acormic, amorphous, etc.) the development of a vitelline placenta by one twin, and of an allantoic by the other, may suffice to explain the retarded evolution of the former and its apparent defeat in what may be called the struggle for intra-uterine life. At the full time the single after-birth may consist of two parts, one vitelline in origin or omphaloidean, the other allantoic or allantoidean; the former is connected with the aorta of the parasite by the mesenteric artery, and the latter with the terminal branches of the aorta of the normal twin by the umbilical arteries, while the two placental circulations are connected together by a sort of third circulation. Manifestly this matter deserves fuller study than I can give it here. 4. Comparative Embryology of the Placenta. Recent years have witnessed many important contributions to the study of the comparative embryology of the placenta. It cannot, however, be said that our knowledge of the subject is yet complete; rather it may be stated that the matter is in a transition stage, for whilst many of the views of the past have had their foundation seriously undermined, the new hypotheses which have been built up can scarcely be looked upon as securely established. Just as the investigations of Hubrecht dealing with the placenta of the hedgehog (120) have resulted in a re-adjustment of many of our ideas regarding the mammalian placenta in general, so special researches into the development of that organ in another type may cause us again to modify the notions that are current. Most of the orders of the Mammalia are characterised by the presence of a placenta during foetal life; in fact, the possession of this organ is the distinguishing feature of the sub-class Eutheria or Monodelphia, sometimes, indeed, called on this account Placentalia. Of the ten orders into which the placental mammals have been divided, seven have been generally recognised as possessing a foetal' placenta with vessels derived entirely from the allantois, while three have been noted as showing a preliminary placenta supplied by vitelline vessels afterwards replaced by a permanent one with allantoic vessels. With re gard to the seven orders with what may be termed for the sake of brevity an allantoic placenta, in two (Sirenia and Carnivora) the placenta is zonary in form; in two (Cetacea and Lemuroidea) it is diffuse; in one (Ungulata) it may be cotyledonary (as in the Ruminants), or zonary (as in the Elephants and Hyracoidea), or diffuse (as in Perissodactyla and some Artiodactyla); in one (Edentata) it may be diffuse (as in the Manidae or Pangolins), zonary (as in the Orycteropidae or Aard-varks), or discoidal (as in the Sloths, Ant-Eaters, and Armadillos); while in one (Anthropoidea) it is constantly meta-discoidal in form. The three orders in which the allantoic placenta is preceded by a temporary vitelline placenta (Rodentia, Insectivora, and Chiroptera) agree in the form of the organ which is discoidal. If, therefore, the five orders with a meta-discoidal or a discoidal placenta be grouped together Anthropoidea, most Edentata, Rodentia, Insectivora, and Chiroptera-it becomes evident that three of them have been shown to be provided with a temporary vitelline placenta, while the two remaining orders in which this has not been demonstrated are the Anthropoidea (including Homo) and most of the Edentata. I hesitate to use similarity in the form |