. The elements of experimental embryology. Embryology, Experimental; Embryology. THE MOSAIC STAGE OF DIFFERENTIATION 233 given rise to epidermis.^ This shows that more cells are capable of ear-formation than normally exert this capacity. This is confirmed for Urodeles by experiments on Amhlystoma. At later stages, when the rudiment has invaginated to form the ear- vesicle, the power of ear-formation is lost by the neighbouring epidermis,^ for if the vesicle is extirpated it is not regenerated. Just as in the case of the limb-field, the ear-field very soon shows a polarisation. If a piece of th
. The elements of experimental embryology. Embryology, Experimental; Embryology. THE MOSAIC STAGE OF DIFFERENTIATION 233 given rise to epidermis.^ This shows that more cells are capable of ear-formation than normally exert this capacity. This is confirmed for Urodeles by experiments on Amhlystoma. At later stages, when the rudiment has invaginated to form the ear- vesicle, the power of ear-formation is lost by the neighbouring epidermis,^ for if the vesicle is extirpated it is not regenerated. Just as in the case of the limb-field, the ear-field very soon shows a polarisation. If a piece of the ear-area of Rana nigromaculata (at the stage when the rudiment is just thickened) is rotated through 180°, the auditory vesicle which sub- sequently develops is reversed. Further, a piece of the ear-area of one side grafted on to the opposite side of the body de- velops with the asymmetry of its side of origin.^ This shows that the rudi- WHP- * - -^—^^ ment was already determined as regards two at least of its axes. The gill-region in Amphibia also constitutes a field in the ectoderm of the embryo. At the early neurula stage in embryos of Amblystoma,^ Rana fiisca and esciilenta, and Bombinator,^ rotation ^ig. 114 of a piece of the gill-area through 180° Ventral view of larva of Boju- is followed by development of the gills ^^'"^^^r in which the ectoderm . , 1 • A \ • J of the gill-region on the left (and operculum m Anura) m reversed gj^e had been rotated through orientation. This shows that the field is 180°. * limit between rotated polarised along an antero-posterior axis. ^^^^; ^\ At the same time, the fact that at this op, operculum. (From Braus, early stage it is still only a field and ,, , • tt 1 XVIII, 1914.) not a spatially and regionally deter- mined rudiment is shown by the capacity of two rudiments grafted together to regulate and give rise to a single normal set of gills (provided that the antero-posterio
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