. The biology of the amphibia. Amphibians. 26 THE BIOLOGY OF THE AMPHIBIA investigators. Some of their more general conclusions may be considered here, for they have an important bearing on the causes of the diversity of structure distinguishing species. At the close of gastrulation the potencies for organ formation are segregated in various parts of the embryo, although there may be no external evidence of this mosaic formation of qualitatively unlike regions. Little or no regulation can occur if one of these regions is removed. Thus, if an area destined to produce a forelimb is dissected awa
. The biology of the amphibia. Amphibians. 26 THE BIOLOGY OF THE AMPHIBIA investigators. Some of their more general conclusions may be considered here, for they have an important bearing on the causes of the diversity of structure distinguishing species. At the close of gastrulation the potencies for organ formation are segregated in various parts of the embryo, although there may be no external evidence of this mosaic formation of qualitatively unlike regions. Little or no regulation can occur if one of these regions is removed. Thus, if an area destined to produce a forelimb is dissected away from an Ambystoma embryo at the time of the appearance remain permanently limbless (Harrison, 1915). In con- sidering the origin of struc- ture one must examine first the origin of potencies. At the very beginning of gastrulation the embryo of the newt is not a mosaic of potential parts. If a piece of ectoderm which would be- come neural plate is trans- planted into the place which would become gills, it develops into gills. A little later in gastrulation the same opera- tion will produce no change, for the presumptive neural plate tissue remains neural plate. If the exchange is made at the beginning of gastrulation between the em- bryos of two species of newts readily distinguished by their color, the tissue which would have become neural plate is molded into gill tissue as before, but the tissue resembles that of the donor species in color and character. During gastrulation, transplanted tissue may be molded by the host embryo (Figs. 12, 13) but this tissue does not lose its specific identity (Spemann, 1928). What is the nature of this molding influence? Spemann and his associates have shown that it emanates from the turned-in dorsal lip of the blastopore, the gray crescent region of the frog's egg. Geinitz (1925) transplanted a piece of this potential of the tail bud, the region will. A B Fig. 12.—The effect of an organizer. A. Neurula of the newt, Triturus taeni- atus,
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