. The elements of experimental embryology. Embryology, Experimental; Embryology. i6o organisers: inducers of differentiation lacks the primitive streak which the upper layer possesses. The lower layer is therefore in the same case as a ventral half of an embryo of an amphibian. The organising action of the primitive streak on the lower layer is shown by the fact that the upper layer is capable of inducing the lower layer to give rise to the fore-gut in the correct position with regard to the notochord, from tissue which would normally not have given rise to fore-gut at all. This is shown by ex
. The elements of experimental embryology. Embryology, Experimental; Embryology. i6o organisers: inducers of differentiation lacks the primitive streak which the upper layer possesses. The lower layer is therefore in the same case as a ventral half of an embryo of an amphibian. The organising action of the primitive streak on the lower layer is shown by the fact that the upper layer is capable of inducing the lower layer to give rise to the fore-gut in the correct position with regard to the notochord, from tissue which would normally not have given rise to fore-gut at all. This is shown by experiments in which an upper and a lower layer are cultivated together in such a way that the primitive streak overlies a region of the lower layer other than that which represents the presumptive fore-gut. ^^ Fig. 77 Induction by organiser in birds. Two blastoderms of the chick grafted together. normal neural plate of upper blastoderm; secondary induced neural plate in upper blastoderm, formed in relation to normal neural plate in lower blastoderm. (From Waddington, Phil. Trans. Roy. Soc. B, ccxxi, 1932.) It is clear, therefore, that the primitive streak is an organiser. It has further been found that it possesses regional differences of potency, both as regards self-differentiating capacities and in- ductive power. Anterior pieces of the primitive streak differentiate into neural tube, notochord, and mesodermal somites; middle pieces produce mesoderm with or without neural tube; posterior pieces never produce neural tube. In other words, there is a gradient in developmental potencies along the primitive streak.^ It should, however, be noted that when portions of primitive streak are cultivated in isolation, they give rise to considerably more than their presumptive fate'^ (fig. 78). * See also Hunt, 1932. - Waddington and Schmidt, Please note that these images are extracted from scanned page images that may have been digitally enhance
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