File:Evolution and disease (1890) (14741017086).jpg

Identifier: evolutiondisease00blan (find matches)
Title: Evolution and disease
Year: 1890 (1890s)
Authors: Bland-Sutton, John, Sir, 1855-1936
Subjects: Diseases Medical genetics Abnormalities, Human Animals Disease Congenital Abnormalities
Publisher: New York : Scribner & Welford
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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Fig. 61.—A Chick with two dichotomized digitsand a supernumerary leg. almost every poultry breeder has seen examples, yetaccessory wings are very rare. One specimen only isknown to me, a dove, preserved in the museum of theRoyal College of Surgeons (fig. 62). This bird has anaccessory wing growing from the lower part of thesternum, 9 ii4 EVOLUTION AND DISEASE. The apparent rarity of supernumerary fore limbs, incomparison with hind limbs, will be dealt with whenconsidering the question of dichotomy as manifested inthe trunk. In cats and dogs dichotomy of the terminal segmentsof the fore and hind-limbs is fairly often seen, and afive-legged dog is one of the usual exhibits at a penny
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Fig. 62.—A Dove with an accessory wing, probablydue to dichotomy. monstrosity-show. An uncommon example is repre-sented in fig. 63 ; it is a sheep with an extra fore-limbattached to the scapula. The terminal segment is re-duplicated. Dissection of the parts seemed to show thatthe abnormal limb was due to dichotomy of the limbaxis, but this explanation is not altogether satisfactory.From supernumerary limbs we may now pass to theconsideration of what are commonly known as, double DICHOTOMY. 5 monsters, and endeavour to show that they arise fromdichotomy, partial or complete, of the trunk-axis of theembryo. Before describing actual specimens it will bewell to adduce facts in support of the view that doubleembryos may arise from a single ovum. In 1869 Haeckelx showed that it was possible, by

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