It is well known that the competence of the prospective ectoderm of early amphi-bian gastrulae for neuralization changes with time and region of the embryo. In chickembryo, Woodside (1937) demonstrated also that the neural competence of the epiblastdecreases with increasing age. However, no experimental data bearing on the regionaldifference in competence have been offered. This report represents an attempt to givean analysis of the reactive capacity of the ectoblast of the chick blastoderm with specialreference to time and space. The grafts of Hensen's node at the primitive streak stage, ranging in size from0.3--0.4 mm square were transplanted to the host blastoderms of different ages, varyingfrom the early head process stage up to 4 somites stage (i. e. corresponding to the deve-lopmental stages of 5, 6, 7 and 8 of Hamburger and Hamilton). The grafts were placed underncath the ectoderm at various levels (i.c. antero-latcral to the node or lateral to thenode and streak) of the host. They were then removed at various intervals from 6--30hours after operation, preserved in Bouin's fixatives and stained with dilute Delafieldhematoxylin. It was observed that neural formation, e.g. neural epithelium, neural plate or neuralfold could be induced in the whole epiblast of host blastoderms of stages 5 and 6. Thereactivity changed quickly during the neural fold formation stage (stage 7), the epiblast atthis stage produced only placodal thickenings, except in a few cases the neural formationwas induced in the anterior epiblast of the blastoderm. The placodal thickening couldalso be called forth in the anterior epiblast of host blastoderm of stage 8 (2--4 somitesstage), but the posterior epiblast was incapable of any response to inductors. Ifl otherwords, the competence for neuralization vanished shortly after the late head fold stage(stage 6--7), and the capacity for placodal thickening response vanished in the 4-somitesstage (stage 8). These results clearly demonstrated that the reactive capacity of the epi-blast to inductors decreased with increasing age of the host at the time of transplanta-tion. The results presented here together with those obtained in our previous findingsshowed that there was no regional difference in neural competence in the epiblast of theprimitive streak stage, but such difference did develop gradually with increasing age ofthe blastoderms. Our data indicated that the frequencies of neural response to inductorsof the epiblast were higher at regions anterior and lateral to the Hensen's node andlower at the posterior region in the blastoderm of stages 5 and 6. The frequenciestended to decrease along a gradient axis in a cephalocaudal direction, and appeared tobe no difference in capacity in the epiblast between the left and right side of the blasto-derm. Finally, we also observed in some instances that when a node graft was transplantedto the posterior border of the pellucida area of the blastoderm, a gut-like epitheliumcould be induced from host germ wall endoderm which had been in contact with thechordal mesoderm of the graft. The possible role of endoderm differentiation uponchordal mesoderm and the significance of their relationship are discussed.

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.