The Determination of the Auditory Placode in the Chick

Abstract

The presumptive ear ectoderm, removed from its normal site and transplanted to the amnio-cardiac vesicle, does not develop into an otic placode unless it comes from an embryo with more than nine pairs of somites. The ectoderm which grows over the place from which the presumptive ear ectoderm is removed is induced to form an otic placode, the size of the placode being smaller the older the embryo at the time of operation. If the wall of the neural tube, including the neural crest, is removed from the region of the ear in embryos younger than the nine-somite stage, an ear may nevertheless be formed. Since the ear ectoderm at this stage is not capable of differentiating when isolated, this result shows that inducing agencies other than the neural material are active at this stage. In some of the operated embryos, the acoustico-facialis ganglion was completely lacking, so this structure cannot be the sole organizer of the ear, as Szepsenwol suggested. If both the wall of the neural tube and the presumptive ear ectoderm are removed, no ear is formed even in stages younger than the nine-somite stage, so that it appears that the non-neural inducing agents are more effective when working on presumptive ectoderm of this age than when working on non-presumptive ectoderm. This suggests that the ear ectoderm is beginning to be determined some time before it acquires the capacity for independent differentiation. The wall of the neural tube, from the ear region, can induce a small ear when grafted under the ectoderm of the anterior part of the head. The evidence suggests that the induction of the ear in normal development is due to the combined action of several structures, of which the wall of the neural tube, and the tissues derived from it (ganglia), is one but not the only one.