. The elements of experimental embryology. Embryology, Experimental; Embryology. THE MOSAIC STAGE OF DIFFERENTIATION 229 Neural Tube Field. Fig. 112 Diagram of an amphibian neurula to show the approximate localisation of the main regional fields as yet dis- covered by experimental analysis. The ism in the pre-mosaic stage, in combining a general determination with an epigenetic mode of development. The arm-field and the leg-field are each of them self-differenti- ating in a general way, and will produce only an arm or a leg, as the case may be.^ But certain details of the development of the fi
. The elements of experimental embryology. Embryology, Experimental; Embryology. THE MOSAIC STAGE OF DIFFERENTIATION 229 Neural Tube Field. Fig. 112 Diagram of an amphibian neurula to show the approximate localisation of the main regional fields as yet dis- covered by experimental analysis. The ism in the pre-mosaic stage, in combining a general determination with an epigenetic mode of development. The arm-field and the leg-field are each of them self-differenti- ating in a general way, and will produce only an arm or a leg, as the case may be.^ But certain details of the development of the field are not independent of the im- mediate environment, the re- mainder of the organism, and in particular, its gradient-field. We have seen that the arm-field of Amblystoma is polarised from its inception. The leg-field also has some polarity from the start, as is revealed when it is grafted heterotopically in an abnormal orientation: the original antero- posterior axis of polarity of the disc persists and becomes that of arrows indicate that the fields are the limb. After rotation of the known to be polarised from their first . appearance. (Origmal.) leg-disc at the origmal site (ortho- topic), however, it acquires a new polarity in relation to that of the body as a whole, the leg-disc has a new antero-posterior axis impressed upon it.^ It may be hazarded that this result is due to the size of the disc rotated not being large enough to cover the whole limb-field. Later, however, the antero-posterior axis of the leg-disc is entirely fixed, but the dorso-ventral axis is not. This is also the condition of the arm-disc at the earliest stage studied (middle gastrula). Rotation experiments with discs at this stage show that the determination of the dorso-ventral axis is dependent on the body of the host. Later still, the dorso-ventral axis of the discs is also determined, and the limb-disc has by then proceeded far on the way to becoming a mosaic of determined subregions.
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