The importance of the posterior midline region for axis initiation at early stages of the avian embryo

Abstract

The avian blastoderm acts during its early stages of development as an integrative system programmed to form a single embryonic axis. Here, I report the results of a variety of transplantation experiments of the midline region at stages X-XII, which were carried out to study their relevance for axis initiation. The results of the experimental series discussed herein emphasizes the importance of the posterior midline region (including the marginal zone and Koller's sickle) for axis initiation. This ability resides mainly at stage X in the posterior side of a narrow midline region, while at stages XI-XII it is exhibited at the region which is located more anterior and lateral to the posterior midline region. This posterior midline region has developmental abilities which allow it to initiate a single embryonic axis and at the same time to prevent other regions that also have such abilities to do so. Therefore, in normal development only one embryonic axis develops in the avian blastoderm. It is proposed that the cells which are important to initiate the avian embryonic axis are concentrated mainly at the region of the posterior midline region. These cells may have organizer properties which determine the initiation site of the axis in the avian embryo.