. Evolution and disease . e case ofthe Two-headed Nightingale. Such specimens occur throughout the vertebrate sub-kingdoms. Two embryo sharksare shown in fig. 66. They areunited in the ventral aspect inthe caudal region; the remainsof a single yolk sac existsbetween the pectoral fins, andserves as additional evidence toindicate their origin from asingle yolk. These sharks, withsome similar specimens, are pre-served in the museum of theRoyal College of Surgeons. Thecatalogue states that a femaleshark was taken in the IndianOcean. When brought on deckand cut up, about thirty youngescaped from th
. Evolution and disease . e case ofthe Two-headed Nightingale. Such specimens occur throughout the vertebrate sub-kingdoms. Two embryo sharksare shown in fig. 66. They areunited in the ventral aspect inthe caudal region; the remainsof a single yolk sac existsbetween the pectoral fins, andserves as additional evidence toindicate their origin from asingle yolk. These sharks, withsome similar specimens, are pre-served in the museum of theRoyal College of Surgeons. Thecatalogue states that a femaleshark was taken in the IndianOcean. When brought on deckand cut up, about thirty youngescaped from the abdomen. Thespecimen lived for two days in abucket of sea-water. Severalexamples of sharks, trout,mackerel, and salmon redupli-cated in this manner have beenrecorded. Rauber has contri-buted some excellent observations on this subject. Hehas been successful in detecting many cases of duplicityof the medullary folds, and the evidence seems to indicatethat, had they continued to develop, double embryoswould have Fig. 66.—Two embryo Sharksjoined ventrally. EVOLUTION AND DISEASE. From examples in which dichotomy gives rise to adouble-headed monster as in the snake (fig. 6j) or thefoal, to more complete forms, such as Ritta-Christina,the Two-headed Nightingale, or the sharks (fig. 66\we pass on to instances in which the bond of union ismerely a narrow fleshy band, as in the Siamese these it is but a step to the origin of separatetwin-foetuses by dichotomy of a single ovum. In allcases of duplex monsters which havecome under my notice, the individualscomposing a double monster were ofthe same sex, and there is goodground for the belief that when twinsare of the same sex and enclosed inthe same membranes they are theproduct of a single ovum. Up to this point we have been con-sidering duplex forms in which thebody on opposite sides of the cleftequal each other in development,extremity of a double- We are now in a position to considerspecimens in which the dich
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectabnormalitieshuman
