. The elements of experimental embryology. Embryology, Experimental; Embryology. FIELDS AND GRADIENTS IN NORMAL ONTOGENY 329 meres of the 2-cell stage.^ Here also may be included those cases in which for reasons at present unknown the heavily yolked egg of fish and of birds may exceptionally possess two blastoderms, and perhaps cases of double monsters in scorpions.^ Twinning in the restricted sense, however, is the result of a dichotomy setting in, not during the earliest stages of cleavage, but during later stages of development, and resulting in definite Fig. 152 Incipient
. The elements of experimental embryology. Embryology, Experimental; Embryology. FIELDS AND GRADIENTS IN NORMAL ONTOGENY 329 meres of the 2-cell stage.^ Here also may be included those cases in which for reasons at present unknown the heavily yolked egg of fish and of birds may exceptionally possess two blastoderms, and perhaps cases of double monsters in scorpions.^ Twinning in the restricted sense, however, is the result of a dichotomy setting in, not during the earliest stages of cleavage, but during later stages of development, and resulting in definite Fig. 152 Incipient twinning mechanically produced. Ventral view of Triton embryo from an egg slightly constricted in pre-gastrulation stage, showing slight anterior doubling. , outer gill-filaments; , inner gill-filaments; oc, inner eyes ; olf, olfactory pits ; * pigment overlying heart-rudiments. (From Spemann, Arch. Entzvmech. xvi, 1903.) fission of one embryo into two or more. Here belong the cases of twinning as found regularly in the armadillo (here resulting in the formation of four or eight embryos),^ occasionally in other mam- mals including man (leading to the production of so-called identical twins), or in birds or earthworms leading to the production of double monsters: experimentally leading to the production of double monsters in the frog after reversal of the tgg, in Fiindulus and trout after subjecting the tgg to cold or oxygen-deficiency ^ Newman, 1923. - Brauer, 1917. ^ Newman, 1917, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Huxley, Julian, 1887-1975; De Beer, Gavin, Sir, 1899-1972. New York [Hafner Pub. Co. ]
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