. Embryology. Embryology; Embryology; genealogy. 80 EMBRYOLOGY possess some organization itself? In addition, the concept implies an organ- izing tissue which possesses some organization itself and also the ability to organize other tissues into nervous structures. Another way of expressing the relationship is to say that the region around the dorsal lip is dominant over the rest of the egg or exerts a control over its development. Comparison of amphibian gastrula and chick primitive streak Having outlined the principles of early embryonic development in the amphibian egg, let us see how they
. Embryology. Embryology; Embryology; genealogy. 80 EMBRYOLOGY possess some organization itself? In addition, the concept implies an organ- izing tissue which possesses some organization itself and also the ability to organize other tissues into nervous structures. Another way of expressing the relationship is to say that the region around the dorsal lip is dominant over the rest of the egg or exerts a control over its development. Comparison of amphibian gastrula and chick primitive streak Having outlined the principles of early embryonic development in the amphibian egg, let us see how they apply to the hen's egg. The presumptive values of the early chick gastrula, called a blastoderm, have been mapped in much the same manner as those of the amphibian egg. In the chick it is the primitive streak, a thickening in the blastoderm (Fig. 37), which serves as a marker of polarity. The anterior end of the primitive streak, for reasons which we shall consider in Chapter 15, is homologous with the dorsal lip of the blastopore of the amphibian gastrula. That is, the presumptive notochord and mesoderm are located around the anterior end of the primitive streak, just as they are found above and to the sides of the dorsal lip of the amphibian blastopore. The primitive streak is not only homologous with the lips of the blastopore but it can also be shown to have the same organizer properties. These prop- erties can be demonstrated by some simple transplantations (Fig. 38). If the. 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 Barth, Lester G. (Lester George), 1905-1979. New York, Dryden Press
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