These studies are an attempt to define the essential tissue interactions that lead to nose, lens and ear formation in the sites these organs normally occupy. The explant experiments in the first paper of this series (Jacobson, '63) demonstrated that endoderm is involved in nose induction and to some extent in lens induction. At early neurula stages the prospective ear epidermis is underlain by prospective heart mesoderm that is inducing ear (Jacobson, '63) and is simultaneously being induced to form heart by the underlying endoderm (Jacobson, '60, '61). It is conceivable that the endoderm has at least an indirect role in ear induction. The endoderm is itself determined at very early stages. Since it is to a large extent a mosaic of future parts at early neurula stages, it would be reasonable that these already determined different regions of the endoderm may be the sources of specifically different inductor substances that would help specify that nose form anteriorly, ear posteriorly, and lens between. The defect and reversal experiments that follow provide information about this.
Nose, lens, and ear, respectively, formed i n embryos deprived of their endoderm at early tail-bud stage 22.
Nose, lens, and ear, respectively, formed in embryos deprived of their endoderm at later tail-bud stage 26.
Nose, lens, and ear, respectively, formed in embryos deprived of their endoderm and mesoderm at early neurula stage 16.