. Embryology. Embryology; Embryology; genealogy. 76 EMBRYOLOGY. Fig. 33. A living salamander neurula just after the primitive nervous system, the neural plate, has formed. The neural plate is folding to form a neural tube, and the large, broad brain region can be distinguished from the narrow spinal-cord region. The demonstration of a single potency is very simple and is shown in Figure 34. The same host is used as in the previous experiment, but in this instance the donor gastrula was allowed to develop into a neurula with a well-formed neural plate. Part of the anterior neural plate is cut o
. Embryology. Embryology; Embryology; genealogy. 76 EMBRYOLOGY. Fig. 33. A living salamander neurula just after the primitive nervous system, the neural plate, has formed. The neural plate is folding to form a neural tube, and the large, broad brain region can be distinguished from the narrow spinal-cord region. The demonstration of a single potency is very simple and is shown in Figure 34. The same host is used as in the previous experiment, but in this instance the donor gastrula was allowed to develop into a neurula with a well-formed neural plate. Part of the anterior neural plate is cut out and transplanted into the flank region of an older embryo. Under these condi- tions this piece of tissue develops and forms a perfect eye in the flank region —and nothing more. No somites or notochord develop as in the previous experiment, in which the presumptive eye of the gastrula was transplanted. Between the stage of the early gastrula and the stage at which the neural plate is present this piece of tissue has become restricted in its potencies and can form eye, and eye only. The rest of the nervous system behaves in a similar fashion, with the result that any part of the brain of the neurula will form only brain when transplanted. It is apparent, therefore, that during gastrulation the entire nervous system becomes limited in its potencies and can no longer develop into other structures. The induction of the primitive nervous system With this analysis of the potencies of the ectoderm as a background we can go on to ask what it is that selects, out of all the possible modes of development of the presumptive neural plate, the potency for nervous sys-. 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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