ABSTRACT
Fragments of chick blastoderm containing the posterior two-thirds or two-fifths of the primitive streak have been cultured, using Spratt’s explantation and carbon particle marking techniques. The stage of embryo at operation ranged from full primitive streak to headfold.
Under these circumstances a forward movement was observed in the posterior half of the primitive streak, giving rise after 15 hours of cultivation to an anterior median protrusion which contains structures plainly derived from the primitive streak (fig. 1) or even a distribution of cell-strands recalling axial organs (fig. 2). After 40 hours of cultivation a tail-bud may arise, obviously without any contribution from the node of Hensen.
We therefore suggest that the shortening of the streak should not be considered merely as a regression but as the consequence of three simultaneous and integrated movements: (a) regression of the node and anterior half of the Streak; (b) a general epibolic movement of the whole blastoderm; (c) a forward movement of the posterior part of the streak, most marked at the posterior end, for which the name of ‘posterior node’ is proposed.
