Representing anterior-posterior (rostral-caudal) development of midline. A. Developmental progression of the neural plate, neural keel, neural rod, and neural tube. The brick line denotes the time period when the anterior portion of the above structure is developing, the purple line represents the same for the posterior portion of the structure. The anterior and posterior part_of children of the structure correspond to the colored lines. Small differences in start times between the anterior and posterior regions are not describable due to the length of the developmental stage. B. Schematic cross section of neural tissue at different anterior-posterior positions at the same stage, representing the different morphologies of structures. The neural plate extends from the anterior end of the organism (the head) to the posterior end (the tail). The neural plate is a flat expanse of cells; it develops into a group of cells with a triangular cross section, the neural keel. The neural keel changes from a triangular cross section to a round cross section as it forms the neural rod. Finally, the neural rod cavitates to form a hole down the center, giving it a donut shaped cross-section. The anterior end develops the characteristics of the next structure, while the posterior end is a few morphological phases slower, e.g. the posterior of the organism is still neural plate when the anterior has started forming a rod morphology.