STUDIES ON THE REGIONAL RESPONSES TO INDUCTOR OF THE CHICK BLASTODERMS OF DIFFERENT AGES / 解剖学报

ABSTRACT

As an extension of previous investigation, the competence of the epiblast was sudiedin further detail, with special reference to the regional response of the chick blastodermto inductor. Transplantation of Hensen's node grafts were performed according to themethods described previously. The host blastoderms were removed and fixed at variousintervals from 19--40 hours after operation. They were then stained with diluted Dela-field hematoxylin, cleared and examined as whole mounts. Most of the specimens weresectioned in series at 8? for further observations. Of a total of 134 operated specimens, 46 showed an induction of secondary struc-tures from the host blastoderms, in various degrees of regional differentiation 37, pro-duced placodal thickenings and the remaining 51 had no response in the blastoderms. It was observed that secondary structures with regional differentiation were inducedexclusively in the epiblast of host blastoderms of stages 5 and 6. The responsiveness ofthe epiblast decreased rapidly in older blastoderms. The latter either produced onlyplacodal thickenings (stage 7--8) or no response (stage 8--9) to inductive stimu-lus. Our experiments demonstrated that the pattern of the induced structures was deter-mined by a regionally different responsiveness of the epiblast of the host blastoderms,accompanied by the process of aging, rather than the specific actions of the inductors.The evidence was that when a head organizer (node graft of stage 4) was transplantedto the host blastoderm, the secondary head structure was induced exclusively in the headregion of the host, and of the secondary trunk-tail structure appeared in the trunk region.In other words, the prospective head epiblast of host blastoderms responsed to the in-ductor by forming a brain structure, whereas the prospective trunk epiblast, responsedto the same inductor (head organizer) by forming a spinal cord. Further evidence wasfound in some instances that the induced trunk-tail structure was often provided withpart of hind brain when the inductors were transplanted to the prospective hind brainregion. In this case, the epiblast of that region responsed to the inductor by forming ahind brain structure. The possible role of patterning of the induced structures upon the responsiveness ofthe epiblast that associated with regional host influence was discussed.