. Analysis of development. Embryology; Embryology. Teleosts and Birds ectoderm that would normally have re- mained extraembryonic was placed in the path of otherwise undisturbed invaginating archenteron roof, resulted in induction of forebrains that were normal in many cases. Some abnormal indvictions resulted, and some undersized forebrains as well. For Fun- dulus, as Figure 116A shows, forebrain and eye differentiate from transplants of anterior embryonic shield pieces even if mesoderm is not included, although better if the under- lying material is present. It should be men- tioned that in
. Analysis of development. Embryology; Embryology. Teleosts and Birds ectoderm that would normally have re- mained extraembryonic was placed in the path of otherwise undisturbed invaginating archenteron roof, resulted in induction of forebrains that were normal in many cases. Some abnormal indvictions resulted, and some undersized forebrains as well. For Fun- dulus, as Figure 116A shows, forebrain and eye differentiate from transplants of anterior embryonic shield pieces even if mesoderm is not included, although better if the under- lying material is present. It should be men- tioned that in Fundulus the anterior end of the notochord lies at the hindbrain level, so that the prechordal inductor must be relatively extensive. The above-mentioned grafts, however, developed in the host em- bryonic shield, and were thus exposed to undefined host induction fields. In the ex- periments cited in the preceding section, where an axis was induced by transplanted dorsal lip, forebrain defects were reported to be of almost imiversal occurrence in the induced axis. Under the hypothesis that the anteriormost cells invaginated are capable of inducing forebrain, specifically, these de- fects would be attributed to the absence of a definite region of the transplanted in- ductor. Less compatible with this hypothesis is the failure (Fig. 116B) of anterior archen- teron roof to indvice eye or forebrain under conditions where posterior levels of the same structure perform appropriate inductions. It will be recalled that a highly effective method of producing cyclopia in Fundulus involves exposure to deleterious agents in early cleavage stages (Stockard, '07). These considerations make it necessary to believe, at the very least, that if the localized in- ductor hypothesis is true, there must also be special additional factors in the anterior tip of the axis, much more easily disturbed by environmental changes than are the rela- tions in the rest of the dorsal axis. In the chick, peculiari
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
