In the united or monstrous twins just described each individual is of equal size and no mistake can be made about their condition, but we now reach a series which is very puzzling in composition, and not easily interpreted, because one of the individuals is dwarfed so that it forms a mere appendage upon a fully grown individual. The human freaks which accompany the travelling showman are usually of this variety. The model of a famous Chinaman is shown in the collection we are at present surveying. Attached to the front of his body immediately below the breast-bone is a curious structure in which may be recognized a trunk, two lower extremities and two dangling processes which apparently represent arms, but where the head should be there is only an indistinct scar. If we suppose one of the Siamese twins to have been arrested in growth at an early stage of fcetal life and become dependent on the other for its blood supply it would have assumed the form of such an appendage as we see on the Chinaman. A year or two ago the writer saw a native of India with a similar malformation. The limbs of the parasitic twin were destitute of movement and of sensation.
Another of the commoner forms of monster is that in which the posterior end of the embryo has been divided, each division of the hinder end of the body being provided with a pair of lower extremities. In many of these cases only one of the two divisions continues to grow, the other being arrested, so that in the adult one may see an extra pair of lower extremities of small size attached to the seat of the normal body or only one of these extra limbs may grow. In such a case the individual may appear to have three legs. In those cases of division of the hind end of the body the sexual parts and anus may be double.
In the case of the Chinaman just described, the head of the atrophic twin had not been developed, but in the specimen which represents the head of the " Bengalee child " we see an opposite condition produced ; a head has developed, with no body attached to it. In this celebrated case the child lived for several years; the additional head was upturned with its crown imbedded in that of the living child. The eyes of the additional head could move. An adjoining specimen shows the true nature of the additional head of the Bengalee child. This specimen shows two twins normally formed except that the heads are joined together crown to crown. One can understand that if the circulation were to be cut off from one of the foetuses, its head might still be supplied from the vessels of the head of the survivor and thus continue to grow while the rest of the body atrophied and disappeared. We see from these experiments which nature has made, and we know now that they are made by the normal condition of development being altered, that the human body during its embryonic and fcetal stages is very plastic and capable of taking on many forms. Monster chicks can be produced by hatching eggs at too high a temperature. If the ova of the sea urchin or of the salmon are hatched in water containing certain salts in solution, many monstrous forms are produced. Medical men have been unable to find evidence to support the widely spread popular tradition that such monsters may be the result of mental shocks or impressions received by the mother during pregnancy. We do not see how such impressions could be conveyed to a foetus in which all the parts are already formed. On the other hand we can see that fever or altered conditions of the mother's blood or uterus might disturb the normal condition of development, and thus produce monsters. Malformations are very apt to occur in certain families, especially those which are regarded as neurotic.
We are now in a position to consider two specimens which made a sensation when first reported, nearly a century ago. They are very similar, so that only one of them need be described. We select the specimen which is part of a lad who died at the age of sixteen from a tumour which was situated within the upper part of the abdomen. Lying within a cavity in the tumour was found the body of a child very similar in shape and size to the one which we saw attached to the epigastric region of the Chinaman. There was a backbone, abdomen, arms and legs, but no head. Here we are apparently dealing with a twin, but one which at an early stage of development became included within the abdomen of the larger or host twin. The " included" twin was hampered in its growth by its peculiar position, and as we have seen, brought about the death of his twin brother.
Another series of specimens appears to illustrate a condition which is allied to the one just described but in reality is very different. They are hollow tumours or cysts which have been excised by the surgeon and may occur in almost any part of the body, but are found most frequently in the reproductive glands. The interior of the cavity seems to be lined with skin, hence they are named " dermoid " tumours or cysts. They often contain tufts or even masses of hair, and it is quite common to find bony plates on their interior to which teeth of various shapes are fixed. It is not easy to explain the occurrence of such peculiar cysts, but it is possible that they arise from a genital cell—one which under normal circumstances would give rise to ova or spermatozoa.
It is now known that the ova of invertebrate animals—such as that of the sea-urchin— may be stimulated to divide and even form an embryo by the application of a particular chemical solution. It is just possible that dermoid cysts may arise from genital cells which have been situated in abnormal surroundings and subject to irregular stimuli.
It has been shown by various embryologists that the cells set aside for reproduction of another generation are separated from the cells which form the embryo at an early stage of development, and that some of them may never reach the testicle or ovary, but become stranded in the body tissues. From such stray cells it is supposed that dermoids may arise, but such an origin is not yet proven.