Heart induction: Distribution of active factors in newt endoderm

Abstract Presumptive heart mesoderm from early neurulae of the newt, Taricha torosa, was combined in explants with endoderm from five different regions of the embryo to study the distribution in the endoderm of the capability to induce heart formation. The percentages of such cultures that ultimately formed beating heart tissue, and the average numbers of days in culture before doing so, are compared with control levels established by explanting presumptive heart mesoderm alone. The ability to induce hearts is found to be broadly distributed and to be strongest in anterior endoderm. The heart‐forming response is accelerated only by the endoderm normally underlying the presumptive heart mesoderm, and this endoderm retains its inductive capability into tailbud stages. Other anterior endoderm is a powerful heart inductor but acts more slowly. Posterior endoderm not only did not promote heart formation, but also increased the time in culture before the beginning of contraction in the few responding cases. Mesoderm immediately posterior to the presumptive heart mesoderm can also be induced to form beating tissue. However, compared to presumptive heart mesoderm, a much longer time elapses before contractions begin. Since endoderm near that which normally underlies the heart has substantial ability to induce heart, the mesoderm overlying these regions experiences some measure of heart induction. Normally these regions of affected mesoderm do not contribute to the heart, but under altered conditions they can. This extended mesodermal area of heart potential is the heart “field.” The existence and extent of the heart field appears to be a result of the distribution of heart‐inducing activity in the endoderm.