The following images illustrate the major changes that occur during normal neurulation of amphibian embryos, as observed from the exterior. The animals we work with are axolotls (Ambystoma mexicanum).  The rapid morphological changes you see in these pictures all occur within three hours, when the embryo is between 63 and 66 hours old!!

All of these pictures were taken by a student enrolled in the 1999 S.E.M. course.

Middle Neurulation

This is a dorsal view of a stage 16 axolotl (Ambystoma mexicanum) embryo.
At this stage of early neurulation, when the embryo is about two and a half days old, the neural folds are moving towards the midline.  Eventually the neural folds will close over the neural plate and fuse, creating a closed neural tube which will give rise to the central nervous system. The anterior end of the neural plate, the wider portion to the left, will form the brain.
Bar = 195 um.

Middle Neurulation

Another dorsal view of a slightly older axolotl embryo at stage 17.
Notice that the nerual folds are in contact in the middle, in the future anterior half of the spinal cord.
The future brain region, to the right, and the posterior region of the spinal cord, to the left, are not yet in contact.
Bar = 195 um.

Late Neurulation

This left lateral view of a stage 18 axolotl embryo shows the neural folds in contact with one another in the region of the future spinal cord.  The folds have not touched in the brain region (to the left).
Bar = 210 um.

Late Neurulation in Cross Section

This beautiful cross section of a late neurula allows you to see the fusing neural folds.  The cells in the neural tube are wedge-shaped and columnar, while the remaining ectodermal cells and cuboidal (seen on the left).
Underlying the neural tube is the notochord which is so clearly seen because if it projection out further than the neural tube.
A mass of paraxial mesoderm (future somite) is apparent directly to the lower right of the neural tube.
Bar = 50 um.