Archive image from page 190 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana0402todd Year: 1849 From a fowl's egq after sixteen or eighteen hours' in- cubation. Magnified four times. a, the germinal area of the cieatricula ; b, the transparent area, containing two primitive traces of embryos; c c, primitive grooves of the double embryonic trace, on each side of which are seen the lamina; dorsales. {After A. Thompson.} upon one yolk, and in one germinal mem- brane or blastodermatic vesicle, there may be formed, in birds, two primitive grooves, which, i
Archive image from page 190 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana0402todd Year: 1849 From a fowl's egq after sixteen or eighteen hours' in- cubation. Magnified four times. a, the germinal area of the cieatricula ; b, the transparent area, containing two primitive traces of embryos; c c, primitive grooves of the double embryonic trace, on each side of which are seen the lamina; dorsales. {After A. Thompson.} upon one yolk, and in one germinal mem- brane or blastodermatic vesicle, there may be formed, in birds, two primitive grooves, which, in their ulterior increment, shall probably form a double monster, as may be seen in a goose's egg, after five days' incubation, represented after Allan Thompson, in. 630. By the for- mation of such a double primitive groove in a single ovum, we may explain the origin of the principal types of double monsters; and on this point a recent observation of Valentin seems particularly worthy of notice, viz. that in which an injury, inflicted on the caudal ex- tremity of an embryo on the second clay, was found, on the fifth, to have produced tbe rudiments of a double pelvis and four in- ferior extremities. But if we admit this cause for those large and principal types, we must acknowledge that it is insufficient for the heteradelphs, and for all those cases in which, the body remaining single, some parts are Double, emliri/o removed from a goose's egg after Jive days' incubation. Magnified four times, g, the common heart; h, rudiments of the superior, i, of the inferior extremities; k, the common ce- phalic fold of the amnios; 7, the caudal folds. (After A. Thompson.) double. For these the excess of formative power is the only explanation we can give. We understand, under this name, not the nisus formrtliviis of the ancient physiologists, working as a Dens ex macliina, but the physical and vital metamorphoses materia?, to which tbe formation of a new being ought to be attri- buted
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