Nature of the hypoblastic influence on the chick embryo epiblast

Stage XIII chick blastoderms deprived of the marginal zone, the area opaca and the posterior half of the hypoblast, when incubated further developed axes whose orientation in 50% of the cases was according to the original blastoderm's orientation, whilst in 50% of the cases they developed at 90° from the posterior side. Those results illustrate the quantitative differences in inductivity between the anterior and the posterior hypoblastic halves. Normally the posterior region has the highest effect but other regions can also bring about the development of an embryonic axis if allowed to act upon the epiblast for a sufficiently long period of time. The possible ways in which a chick hypoblast influences the epiblast to develop an embryo are examined in the light of recent findings and of new experiments described below.