By manually tipping embryos under a dissecting microscope so that their vegetal plates are parallel to the coverslip, we can actually see that a ring of apically constricted cells does form as the vegetal plate begins to invaginate. By using a laser microbeam, we can ablate various populations of cells in the vegetal plate in "tipped" embryos to assess which cells are necessary for invagination. Beth Laxson in our laboratory has ablated bottle cells and shown that they are necessary during the initiation of primary invagination. Below is a 3-d graph of the depth of invagination in an embryo in which a 120 degree arc of bottle cells has been ablated. Note the the ablated region doesn't invaginate nearly as far. This is consistent with a role for bottle cells in initiating invagination. Click on the picture to see a movie of this graph that allows you to rotate to see which regions are less invaginated.