File:Evolution and disease (1890) (14577388668).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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he has never found one ofthese eggs giving rise to a single embryo. It is truethat a single worm escapes from a capsule, but thennearly always the remains of its companion are found.The accompanying sketches exhibit the extreme formsof these double embryos. In fig. 64 A, they are shownwhen of equal size ; in B, one of the embryos hasundergone suppression. We have direct evidence among vertebrata that twoembryos may arise from a single ovum. It has beenactually witnessed in a batrachian by Clarke.2 In the 1 Quart. Journal of Micros, Science, vol. xix. 1879. 2 Ann. Mem. Boston Soc. Nat. Hist/ 1880. DICHOTOMY, 119 spring of 1879 he had in an aquarium two or threethousand eggs oi Amblystomapunctatum for the purposeof studying their development. One day he chanced tofind one with the medullary folds nearly completed, butthey had not united at the cephalic end, and appeared tobe much rounded at their anterior ends, instead of havingthe ordinary vague outlines ; he kept it apart, therefore,
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tJ Fig. 65.—A two-headed Foal; anterior dichotomy. and watched it. Each free portion of the medullary folddeveloped a perfect head, which, at first partly united,became gradually more so, until they were connectedthroughout their entire length. Posterior to the headsthere was no sign of duplicity. In this case a two-headed monster, with a regularsymmetrical body, was developed from one egg, and 120 E VOL UTION AND DISEASE. the anterior portion of each medullary fold gave rise toa head. Double embryos vary greatly, according to the degreeof dichotomy, and the subsequent growth of each half ofthe ovum. The cleavage may affect the cephalic ex-tremity only ; this is conveniently called anteriordichotomy. Of this the two-headed foal sketched infig. 65 will serve as an example. In many cases the cleavage only involves the facialportion of the skull, thus producing an animal with twotongues and four pair of jaws. The supernumeraryjaws are, in such cases, conjoined into a single masswedged

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