Terata, more properly so called, are divided into single, double, and triple monsters. Single monsters may be autositic, or independent of another embryo or foetus; or they may be omphalositic, that is, dependent upon another embryo or foetus, which is commonly well developed, and which supplies blood for both through the umbilical vessels. When an omphalosite exists, the other foetus is called, in this case also, the autosite.
The first order of autositic single monsters contains four genera with eight species, and under these species are thirty-four varieties. They may have imperfect limbs, no limbs, one eye in the middle of the forehead (cyclops), fused lower limbs (siren), and so on. Some of these monsters show a strong resemblance to lower animals, but there is no record that is in any degree scientific of a hybrid between a human being and a lower animal.
There are two genera of the omphalositic single monsters, with four species. One of the twins, the autosite, is commonly a normal child; the other, the omphalosite, may be as small as a child's fist, and be very much deformed. Of these omphalosites the paracephalus has an imperfect head, commonly no heart, and the lungs are absent or rudimentary. The acephalus has no head, and commonly no arms; the asomata is a head more or less developed, with a sac below containing rudiments of the trunk organs. The Acephalus is very rare—the rarest of all monsters except the Tricephalus. There is a fourth kind—the foetus anideus. This is a shapeless mass of flesh covered with skin. There may be a [{76}] slight prominence with a tuft of hair on it at one end of the mass to indicate the head. In this monster there are more traces of bodily organs than might be expected. These four kinds of omphalosites are either dead when born, or they die as soon as the placental circulation is cut off. If there is any probability of life, the physician should give them baptism before the placental circulation is stopped.
Nothing satisfactory is known concerning the etiology of single monsters. Landau, and other authorities as great as he is, reject the theory that maternal impressions from fright or exposure to the sight of hideous deformity are the cause of terata. I think the father is accountable for terata as often as the mother is. Barnes, an English physician, and others claim they find that terata are frequent in consanguineous marriages, but I have not been able to verify the assertion.
It seems a theory may be offered to explain the single terata. In 1888 Roux of Breslau by puncturing one blastomere of a frog's egg in the two-cell stage killed the punctured blastomere without affecting the other. The punctured blastomere remained inactive, but the other developed into a complete half embryo.
Crampton by separating and isolating the blastomeres in the two-cell stage obtained a half embryo; and Zoja by isolating blastomeres of the medusae, Clytia and Laodice, got dwarfed larvae.
Wilson succeeded by the separation through shaking of the blastomeres in the two-cell and four-cell stages in developing Amphioxus larvae, which were half the natural size for the two-cell blastomeres, and commonly half the normal size from the four-cell blastomeres, yet in the latter some of the larvae were of the normal size but imperfect. From the eight-cell stage he got only imperfect larvae. Similar results were obtained by other operators with various eggs.
Driesch and Morgan by removing part of the cytoplasm from a fertilized egg of the ctenophore, Beroe, produced imperfect larvae showing certain defects which represent the parts removed.
In these cases of injured and isolated blastomeres we have, it seems to me, a plausible theory for the etiology of single terata. The blastomeres in the human ovum may perhaps be injured in part by toxins from the mother, or they may be defective through disease in the ovum or the spermatozoon. They also may possibly be displaced traumatically, but this seems to be doubtful.
There are three theories concerning the origin of omphalositic [{77}] terata. Ahlfeld (Missbildungen des Menschen, Leipsic, 1882) holds that the autosite is stronger than the omphalosite, and as a consequence the foetal circulation in the omphalosite is reversed, and development is thus checked. Dareste (Production artificielle des monstruosités, Paris, 1876), Panum (Beitrag zur Kenntniss der physiol. Bedeut. der angeboren Missbildungen, Virchow's Archiv., 1878), Perls (Lehrbuch der allgem. Pathologie) and Breus (Wiener med. Jahrbuch, 1882) maintain there is an inherent original defect in the omphalositic child which prevents development of the blood-vessels, and that Ahlfeld's theory of an indirect umbilical connection of the omphalosite to the placenta is not probable; if it were, omphalosites would be very common, because one of twins is nearly always stronger than the other. Hirst and Piersol (op. cit) combine these theories. This kind of monster is certainly an imperfectly developed human individual, and even the Foetus Anideus should receive at the least conditional baptism.
The next group comprises the composite monsters. Normal twins may arise from the fertilisation of one ovum and of two distinct ova. In 506 cases examined by Ahlfeld he found that 66 twin births came from single ova. Twins from a single ovum are always of the same sex, and they are not easily distinguished one from the other. Triplets may arise from one, two, or three ova. The elder Saint-Hilaire thought that composite monsters arise from the fusion of two impregnated ova, but this opinion is now generally rejected. Composite terata in every instance arise from a single ovum.
There is a divergence of opinion, however, as to the origin of a composite monster in the single ovum. Some authorities maintain that these monsters arise from the union of two originally separate primitive traces. This supposes primitive duality followed by fusion (Verwachsungstheorie). Other writers hold that there is originally one primitive trace, and that composite terata are the product of a more or less extensive cleavage of this single blastoderm. This supposes primitive unity followed by fission (Spaltungstheorie). Here, as in the case of normal development, the argument is founded on analogy. The earliest stage in the development of a human double monster observed was at the fourth week after fertilisation—Ahlfeld's case.
B. Schultze (U. anomale Duplicität der Axenorgane, Virchow's Archiv.) and Panum and Dareste (op. cit.) hold the fusion theory— [{78}] the fusion of two separate blastoderms in one ovum. Panum and Dareste have seen two separate normal blastoderms on one ovum. Allen Thompson in 1844 (London and Edinburgh Monthly Journal of Medical Science), Wolff, von Baer, and Reichert also observed two embryos in one ovum. Dareste is of the opinion that the fusion of two separate ova is impossible. The fission theory—the fission of a single blastoderm to make a composite monster—is supported by Wolff, J. F. Meckel, von Baer, J. Müller, Valentine, Bischoff, and others, especially by Ahlfeld. Ahlfeld says that this single blastoderm is split by pressure.
Gerlach also (Die Entstehungsweise der Doppelmissbildungen, etc., Stuttgart, 1882) admits fission, but he contends that it is not so simple a process as Ahlfeld thinks it is. It is not a passive cleavage, but a result of a force in the cell-mass existing before differentiation. Gerlach calls fission at the anterior or head-end of the single blastoderm, bifurcation; and he has actually observed such bifurcation in a chick embryo of sixteen hours (U. d. Entstehungsweise der vorderen Verdoppelung. Deutsche Archiv. f. klin, Med., 1887). In this case the first change noticed was a broadening of the anterior end of the primitive streak; next a forked divergence appeared, and this became more pronounced; until by the twenty-sixth hour the bifurcation was half as long as the undivided posterior part. From each anterior end of the diverging branches a distinct head-process extended. Allen Thompson (loc. cit.) in 1844 saw a goose-egg, which had been incubated for five days, in which was a double monster divided to the neck.
Beyond this observation by Gerlach we have the fact, which seems to make for the fission theory, that no matter how unequally nourished or how variable in extent, the union between the halves of double monsters is always symmetric—exactly the same parts of each twin are joined. This seems to exclude a fortuitous growing together of dissimilar areas or cell-masses, for non-parasitic double terata at the least. Born ( U. d. Furchung des Eies bei Doppelbildungen, Breslauer Aerztl. Zeitschr., 1887), in a study of fish ova, found that ova which produce double monsters begin with a segmentation like that of the single normal ovum.
If fission is complete homogeneous twins are the result; these twins are of the same sex and very similar in appearance. Incomplete fission, as has been said, gives rise to double or triple terata. If one of the teratic twin embryos is stronger than the other, the various combinations of enclosure and parasitism may result, although the origin of parasitic double terata is not convincingly clear. A triple [{79}] monster, according to the fission theory, arises from a double incomplete cleavage of the primitive trace. Dr. Ephraim Cutter has observed teratic composite spermatozoa which, he thinks, probably have influence in producing composite monsters.
There are three orders of the double autositic monsters: Terata Katadidyma, in which the embryonal fission was at the cerebral end; the Terata Anadidyma, divided below; the Terata Anakatadidyma, divided above and below, but joined at the middle of the body. There are four genera of the Terata Katadidyma with many species. The first genus is the Diprosopus, the double-faced. The doubling varies from the finding of two complete faces to a slight trace of duplex formation in one head. Förster in 500 human monsters observed 29 cases of diprosopi.
There are six species of diprosopi: 1. D. Diophthalmus, which has only two eyes, but there is a doubling of the nose. 2. D. Distomus, which has two mouths, two lower jaws, two tongues, one pharynx, and one oesophagus. 3. D. Triophthalmus, which has three eyes, and the doubling of the face is more complete. There are only two ears. 4. D. Tetrophthalmus, which has four eyes and two well-separated faces. 5. D. Triotus is like the last, but it has three ears. 6. D. Tetrotus has four ears, four eyes, and there is some doubling at the pharynx. Two oesophaguses enter one stomach in this species commonly. D. Tetrotus is rare—only one example in man is known. In all diprosopi there is only one trunk, one pair of arms, and one pair of legs. Sir James Paget had a photograph, made in 1856, of a living diprosopus, the second face of which had a mouth, nose, eye, part of an ear, and a brain (?) of its own. The two faces acted simultaneously, suckled, sneezed, yawned together.
Are diprosopi twins? An answer to this question will be clearer after a description of other composite terata.
The second genus of the Terata Katadidyma is the Dicephalus. This genus comprises five species, which have in each case two heads, with separate necks commonly. There are two vertebral columns, which usually are separate down to the sacrum, and they converge at the lower end. [{80}] In the interior organs doubling will be found corresponding to the degree of separation of the trunks. In all the species of this genus there are one umbilicus and one cord.