On other shelves of the teratological gallery are shown numerous specimens to illustrate the developmental abnormalities of hare-lip, of cleft palate, and of other malformations which have already been alluded to. Nor need those ugly births detain us, in which the top of the head is absent, giving the child a frog-like appearance ; nor those in which the spinal cord is exposed as a flat plate along the back. But there are two very curious conditions which may have some interest for the reader. One of the conditions is that of cyclops. The two eyes are fused together, forming a single eye under the middle of the forehead. The condition is one which never could have existed at any stage of human evolution. To understand its origin one must first look at another series of specimens in which the two lower extremities are fused together and stretched out so as to form a continuation of the axis of the trunk. One can see very well how the hind limbs became fused together in embryonic life. They appear first as small flattened buds on the side of the posterior part of the embryonic body. Now if the posterior end is arrested in development the two limb buds appear side by side in their normal position, but being unseparated by the hinder end of the body they fuse together across its hinder end. Arrest of growth may occur at the anterior end of the embryonic body—the most anterior part being that which separates the eyes and forms the middle part of the nose. Hence if an arrest of growth of the middle or nasal part of the face occurs the eye buds are unseparated and they fuse together in the earlier stages of facial development. The anterior ends of the hemispheres of the brain are also joined to some degree. Thus we see that the posterior and anterior ends of the embryo may be arrested in their development, with the result that the hind limbs fuse in the one case, the eyes in the other. The one condition is known as sympodia, the other as cyclops.
In cases of a human cyclops there is often present a small appendage shaped like the proboscis of an elephant. The appendage is situated above the median eye and represents the rudiments of the parts which are normally incorporated in the development of the nose. We cannot suppose that any human ancestor was normally provided with a free proboscis and median eye.
The second abnormality to which we would draw the reader's attention relates to the lower jaw or mandible. It is not uncommon to see a face sadly deformed by an arrest of growth of the lower jaw. Instead of projecting prominently, the chin and lower lip are drawn downwards and backwards so that the face appears to merge into the neck without any sharp boundary between them. The museum specimens illustrate very pronounced examples of this deformity. The arrest in the development of the mandible and of the tissues formed with it from the first visceral arch of the neck, is almost complete. The ear holes, evolved from the upper part of the first visceral (gill) clefts are drawn downwards so that they almost meet in front of the neck. It is possible that this condition may be a reversion, for the lower jaw is very poorly developed in several of the most primitive types of fishes. It is worthy of remark that this condition—known as agnathia—may occur in all the domestic breeds of animals and is especially frequent in South Down sheep.
We will bring this brief survey of monsters and malformations to a conclusion by noting a few of those congenital deformities which affect the limbs. The fusion of the hind limbs (sympodia) and congenital club foot have been already mentioned and need not be further described. It has also been mentioned that the limbs appear as small flat outgrowths from the side of the trunk at the end of the third week of development, when the embryo is only about a tenth of an inch in length. It is a week later before the three limb segments are differentiated—those of the upper arm and thigh, of the forearm and leg, and of the hand and foot. The buds of the upper and lower extremities are at first very similar in appearance. The fingers and toes, when they first appear, are imbedded in the flattened or webbed extremities. Often some degree of webbing persists, and with this condition some of the joints in the fingers or toes may fail to form. Such abnormalities are apt to be hereditary, and to run in families. The limb buds may be arrested in growth at an embryonic stage, and when the individual is quite adult all that may be seen of them are minute tags of skin. It is fortunate for us that such severe errors or diseases of development are of rare occurrence. One of the commonest departures from the normal is the presence of an extra digit. Supernumerary digits may be situated at the little finger or thumb side of the hand. It is possible that the occurrence of a sixth digit may be due to the reappearance of an ancestral condition, but if so it must represent a very early stage in the evolution of the hand and foot, for in all the higher vertebrates the number of digits does not exceed five. Man appears to retain normally the original complement. It is very possible that the extra digit is due to a division or dichotomy of the bud for the little finger or thumb. We have seen that one end of the embryo may divide so as to form a double individual, and it is also probable that this may happen in the bud which gives rise to a finger or toe.
Before leaving this subject we may glance at a form of abnormality which illustrates the fact that when reversions occur in man they usually relate to a very remote stage of evolution. The abnormality to which we wish to draw the reader's attention is a bony process—the supracondyloid process—which is attached to the inner side of the humerus a few inches above the elbow joint. It occurs in one body out of every fifty examined. The preparations show that the main artery and nerve of the upper arm pass under this hook-like process. The supracondyloid process appeared in the humerus of the ancestors of modern mammals; it is seen in many reptiles; the lemurs have it, so have the carnivora and many other forms; but it is absent in all the higher primates except as an abnormality. Its use we do not know, but it is remarkable that this trace of a pre-mammalian stage should so often appear in man. Sir John Struthers showed that it passed from father to son, and one can conceive that it may have persisted in some individuals at every stage of man's evolution.
Mention has already been made of the anomalous position of man's great toe, and of the fact that no case of complete reversion to the anthropoid condition has been observed either in monstrous or normal human births. The evidence that the human great toe was at one time thumb-like and separated from the other toes is, however, convincing, and we have to see how the human condition could have been evolved. It has just been mentioned that when the hand and foot are first formed all the digits are united together or webbed, as is the case in all mammals. The separation of the thumb and great toe is a secondary process. The great toe of man appears to have been evolved by a retention of the condition seen in the embryonic limb. In other words, we may regard the great toe of man, as far as its relationship to other digits is concerned, as having reverted to the primitive mammalian position.