Consider a Sheet of Epithelial Tissue That Invaginates to Form

Short Answer

Consider a sheet of epithelial tissue that invaginates to form a tube along the anterior-posterior axis of a developing vertebrate embryo. The tube is then closed concomitant with a "convergence and extension" process similar to that observed in germ-band extension in Drosophila melanogaster. During this process, the tube elongates and narrows before pinching off from the sheet. Which axis (1 or 2) in the following schematic drawing of the tube cells do you think corresponds to the anterior-posterior axis? Write down 1 or 2 as your answer.