(12) PART 2: Double Monsters (Siamese Twins, etc.)
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 by a thick fleshly ligament from the lower end of the breast-bone (xiphoid cartilage), having the common navel on its lower border; the anatomical examination showed, however, that a process of peritoneum 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 ligament has been but 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 fusions of two individuals into one down to the truly marvelous condition of a small body or fragment parasitic upon a well-grown infant, -- the condition known as foetus in foeto.
These monstrosities are deviations, not from the usual kind of twin gestation, but form a certain rarer physiological type of dual development. In by fair the majority of cases twins have separated appendages, and have probably been developed from distinct ova; but in a small proportion of (recorded) cause there is evidence, in a placental and enclosing structures, that the twins had been developed from two rudiments arising aside 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 hens egg at the sixteenth or eighteenth hour of incubation two "primitive traces" or rudiments of the backbone forming side by side; and in a gooses egg incubated five days he found on one blastoderm two embryos, each with the rudiments 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 traces as to irregular budding from the margin of the germinal disc. In any case, the perfect physiological type appears to have two rudiments on the blastoderm, whose entirely separate development 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 distinct primitive traces, the uni-axial type was approximately reverted to in the process of development, as in the formation 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 fragmentary foetus is attached to or enclosed in a foetus of average development, -- the latter class being the so-called cases of "parasitism."
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Abnormal Development & Congenital Malformations - Table of Contents