MONSTER 765 tions, there are others of them referable to the effects of mechanical pressure, and even to direct amputation of parts within the uterus. Acardiac and Acranial Monsters. It sometimes happens in a twin pregnancy that one of the embryos fails to develop a heart and a complete vascular system of its own, depending for its nourishment upon blood derived from the placenta of its well-formed twin by means of its umbilical vessels. It grows into a more or less shapeless mass, in which all traces of the human form may be lost. Other viscera besides the heart will be wanting, and no head distinguishable ; the most likely parts to keep the line of development are the lumbar region (with the kidneys), the pelvis, and the lower limbs. The twin of this monster may be a healthy infant. Reversed Position of the Viscera. This is a develop mental error depending on the retention of the right aortic arch as in birds, instead of the left as is usual in mammals. The position of all the unsymmetrical viscera is transposed, the spleen and cardiac end of the stomach going to the right side, the liver to the left, the caecum resting on the left iliac fossa, and the sigmoid flexure of the colon being attached to the right. This condition of situs inversus viscerum need cause no inconvenience ; and it will probably remain undetected until the occasion should arise for a physical diagnosis or post-mortem inspection. There are numerous other anomalies in the development of the great vessels. In the heart itself there may be an imperfect septum ventriculorum, and there is more frequently a patency of the foetal communication between the auricles, permitting the venous blood to pass into the arterial system, and producing the livid appearance of the face known as cyanosis. The causes of congenital anomalies are difficult to specify. There is no doubt that, in some cases, they are present in the sperm or germ of the parent ; the same anomalies recur in several children of a family, and it has been found possible, through a variation of the circumstances, to trace the influence in some cases to the father alone, and in other cases to the mother alone. The remarkable thing in this parental influence is that the malformation in the child may not have been manifested in the body of either parent, or in the grandparents. More often the malformation is acquired by the embryo and foetus in the course of development and growth, either through the mother or in itself independently. Maternal impressions during pregnancy have often been alleged as a cause, and this causation has been discussed at great length by the best authorities. The general opinion seems to be that it is impossible to set aside the influence of subjective states of the mother altogether. The doctrine of maternal impres sions has often been resorted to when any other explana tion was either difficult or inconvenient ; thus, Hippocrates is said to have saved the virtue of a woman who gave birth to a black child by pointing out that there was a picture of a negro on the wall of her chamber. Injuries to the mother during pregnancy have been unquestion ably the cause of certain malformations, especially of congenital hydrocephalus. The embryo itself and its membranes may become the subject of inflammations, atrophies, hypertrophies, and the like ; this causation, to which Otto traced all malformations of the fcetus, is doubtless accountable for a good many of them. But a very large residue of malformations must still be referred to no more definite cause than the erratic spontaneity of the embryonic cells and cell-groups. The nisus formativus of the fertilized ovum is always made subject to morpho logical laws, but, just as in extra-uterine life, there may be deviations from the beaten track ; and even a slight deviation at an early stage will carry with it far-reaching consequences. This is particularly noticeable in double monsters. 2. Double Monsters. Twins are the physiological analogy of double monsters, and some of the latter have come very near to being two separate individuals. Triple monsters are too rare to dwell upon, but their analogy would be triplets. The Siamese twins, who died in 1874 at the age of sixty, were joined only by a thick fleshy ligament from the lower end of the breast-bone (xiphoid cartilage), having the common navel on its lower border ; the anatom ical examination showed, however, that a process of peri toneum extended through the ligament from one abdominal cavity to the other, and that the blood-vessels of the two livers were in free communication across the same bridge. There are one or two cases on record in which such a liga ment has been cut at birth, one, at least, of the twins surviving. From the most intelligible form of double monstrosity, like the Siamese twins, there are all grades of fantastic fusion of two individuals into one down to the truly marvellous condition of a small body or fragment parasitic upon a well-grown infant, the condition known as foetus in fcetu. These monstrosities are deviations, not from the usual kind of twin gestation, but from a certain rarer physiological type of dual development. In by far the majority of cases twins have separate uterine appendages, and have probably been developed from distinct ova ; but in a small proportion of (recorded) cases there is evidence, in the placental and enclosing structures, that the twins had been developed from two rudiments arising side by side on a single blastoderm. It is to the latter physiological category that double monsters almost certainly belong; and there is some direct embryological evidence for this opinion. Allen Thomson observed in the blastoderm of a hen s egg at the sixteenth or eighteenth hour of incubation two " primitive traces " or rudiments of the backbone form ing side by side ; and in a goose s egg incubated five days he found on one blastoderm two embryos, each with the rudi ments of upper and lower extremities, crossing or cohering in the region of the future neck, and with only one heart between them. Somewhat similar observations had been previously published (four cases in all) by Wolff, Von Baer, and Reichert. Malformations in the earliest stages of the blastoderm have been more frequently observed of late, especially in the ova of the pike ; and these point not so much to a symmetrical doubling of the primitive trace as to irregular budding from the margin of the germinal disc. In any case, the perfect physiological type appears to be two rudiments on one blastoderm, whose entirely separate de velopment produces twins (under their rarer circumstances), whose nearly separate development produces such double monsters as the Siamese twins, and whose less separate development produces the various grotesque forms of two individuals in one body. There can be no question of a literal fusion of two embryos ; either the individuality of each was at no time complete, or, if there were two dis tinct primitive traces, the uni-axial type was approximately reverted to in the process of development, as in the forma tion of the abdominal and thoracic viscera, limbs, pelvis, or head. Double monsters are divided in the first instance into those in which the doubling is symmetrical and equal on the two sides, and those in which a small or fragment ary fcetus is attached to or enclosed in a fcetus of average development, the latter class being the so-called cases of "parasitism." Symmetrical Double Monsters are subdivided according to the part or region of the body where the union or fusion exists head, thorax, umbilicus, or pelvis. One of the simplest cases is a Janus head upon a single body, or there may be two pairs of arms with the two faces. Again, there
may be one head with two necks and two complete trunks