The Initiation of Gastrular Ingression in the Chick Blastoderm

Normal gastrular ingression in the chick blastoderm occurs in two steps. The first consists in de-epithelialization of the cells in the middle of the young primitive streak. The cells that will ingress converge as a sheet towards the primitive streak; this convergence builds up the elongating primitive streak. These cells come from a large posterior area of the area pellucida. In this area they show many blebs at their ventral side. These blebs are not visible in the more lateral regions of the upper layer at this stage. During the second step of ingression, de-epithelialization goes on in the middle of the primitive streak, but convergence within the upper layer has come to an end, while migration of the ingressed middle layer cells starts, away from the primitive streak. To observe the first stages of ingression, we studied secondary primitive streaks, induced by grafting a nodus posterior into the entophyllic crescent of a host blastoderm. We fixed blastoderms in which, though a secondary primitive streak was not yet visible, spreading of the graft had taken place so as to make evocation of a streak most probable. From this study we conclude that the initiation of de-epithelialization in experimental and probably in normal chick gastrulation is not preceded by an overall lysis of the basal lamina at the future site of ingression. Ingression starts and goes on as a de-epithelialization of individual cells.