Focus Article Internalizing the vegetal cell mass before and during amphibian gastrulation: vegetal rotation and related movements Rudolf Winklbauer ∗ and Erich W. Damm The movement of the prospective mesoderm and endoderm to the interior of the amphibian embryo starts in the vegetal cell mass well before the onset of overt gastrulation. By an animally directed movement of cells, the vegetal mass constricts its outer part and expands its inner region including the blastocoel floor, in a process of pregastrulation emboly. Further internalization of the vegetal region has been studied in the Xenopus embryo. At the onset of gastrulation, vegetal rotation sets in at the periphery of the vegetal cell mass, first dorsally and then spreading laterally and ventrally. It consists of an intense inward surging of cells due to active cell rearrangements that can be observed in explants of the vegetal cell mass. In its course, the blastocoel floor expands further and becomes apposed to the blastocoel roof. The boundary between apposed floor and roof forms Brachet’s cleft. Another effect of vegetal rotation is the downward and inward movement of the mesodermal marginal zone, constituting the first phase of involution. Together, the upward and outward movement of the peripheral vegetal mass and the downward and inward translocation of the marginal zone lead to an apparent rotation of the whole peripheral region of the gastrula. Vegetal rotation continues to contribute to endoderm internalization to near the end of gastrulation.  2011 Wiley Periodicals, Inc. How to cite this article: WIREs Dev Biol 2012, 1:301–306. doi: 10.1002/wdev.26 INTRODUCTION T he essence of gastrulation is the movement of the mesodermal and endodermal germ layers to the interior of the embryo, and to this end, a variety of mechanisms are employed in different groups of animals. A common mechanism in invertebrates is invagination, the inward bending of a single- layered epithelium, and a close relative of vertebrates, Branchiostoma, gastrulates by invagination. 1 A characteristic of vertebrates is the multilayered structure of the blastula wall. In particular, in vertebrates with primary holoblastic cleavage, such as lampreys, sturgeons, lungfishes or amphibians, the prospective endoderm of the lower half of the blastula ∗ Correspondence to: r.winklbauer@utoronto.ca Department of Cell and Systems Biology, University of Toronto, Toronto, Ontario, Canada consists of a huge, multilayered vegetal cell mass (Figure 1(a)). This feature may preclude mesoderm and endoderm internalization by invagination. In fact, invagination seems to be reduced in these groups to the initial indentation of the blastopore due to bottle cell formation 2 (Figure 1(a)). In its place, other processes drive internalization. In the frog, Xenopus laevis, the mesodermal marginal zone above the blastopore lip (Figure 1(a)) involutes. 3,4 The endodermal vegetal cell mass (Figure 1(a)) actively moves to the interior by vegetal rotation, 4–7 while convergent extension of the dorsal axial mesoderm shifts the marginal zone vegetally and thus contributes to its passive engulfment. 8,9 Moreover, in Xenopus 6,7 as in urodeles, 10–12 a characteristic shape change of the vegetal cell mass suggests an inward translocation of vegetal material already at blastula stages. Volume 1, March/April 2012  2011 Wiley Periodicals, Inc. 301