Induction by the Primitive Streak and Its Derivatives in the Chick

IT has recently been shown (Waddington, 1932) that in the chick embryo a developing primitive streak can cause any ectoderm lying above it (provided only that the ectoderm is of the right age) to differentiate into a neural plate. It was shown that in all successful cases the grafted primitive streak had yielded both neural tissue and mesoderm, but that in some specimens the grafted neural tissue was very small in amount and was not in contact with the host ectoderm, and from this fact it was concluded that the mesoderm is capable of acting as an organiser in the absence of neural tissue. The first specimen to be described in this paper confirms this conclusion, since in this case the grafted tissue has given rise only to mesoderm, and this uncontaminated mesoderm has induced the formation of a neural plate from the host ectoderm. The remaining specimens which will be described illustrate two main theses: firstly, that a capacity for induction is retained for some time by the derivatives of the primitive streak, and secondly, that this capacity is still possessed by the differentiated neural tissue at a stage when it appears to have been already lost by the notochord. The technique used was similar in all respects to that described in the paper mentioned above. The embryos were fixed in Bouin's fluid, sectioned at 15/x and stained with Delafield's haematoxylin.