Quelques précisions sur la cinématique de la ligne primitive chez le Poulet

Abstract

ABSTRACT Isolated anterior and posterior fragments of very young blastoderms at the primitive-streak stage, transected in front of the streak, have been cultured for 24 hours. The posterior fragments were marked by carbon particles. Our results stress the importance, during bird gastrulation, of the transition region between the area opaca and the area pellucida, especially at the site of formation of the primitive streak. Considering this region as an extension centre, forward and backward, of the primitive streak enables us to understand the radically opposed opinions of Pasteels and Spratt by furnishing illustrations of the influence of in vitro culture circumstances. We have seen, indeed, that under Spratt’s marking conditions on whole blastoderms, absence of epiboly practically inhibits all forward movement in the blastoderm. On the contrary, however, this last tendency is seen very clearly in our experiments where, by releasing it from the hindrance formed by the anterior part of the blastoderm in vitro, we enable the streak to stretch forward. Our results, therefore, confirm the findings of Pasteels with regard to his controversy with Spratt and show that the anterior end of the very young primitive streak already contains the material of the future Hensen’s node. Using Spratt’s technique we arrive at a confirmation of Pasteels’s opinions based on in ovo staining. It seems worth while to note that our experiments and those of our predecessors tend to localize the ‘organization centre’ in the region where earlier the formation of the endophyll started and as a consequence its contact with the ectophyll is the most precocious and the most intimate. This is stated without any implication as to the exact mode of formation of the inner germ layer. On the other hand, this point in the midst of the primitive streak is the point of convergence of Hensen’s node and the posterior node, and the place where the tail-bud will arise, as we have previously shown (Vakaet, 1960).