Yesterday we spied a new clutch of frog eggs in our pond. A check under the microscope revealed that the embryos were a good deal younger than we had previously seen. They were at the neurula stage, which is when the brain and spinal cord first starts to form.
I removed an embryo from the jelly mass and took an image every 10-20 minutes to reveal the dynamics of this amazing process. In the sequence below, we are looking down on the future back of the embryo. The head end of the embryo is towards the bottom. The embryo is about 1 mm long.
To help you understand what is happening, I've pulled out images 3, 10 and 18 and annotated them below.
The early stages in the formation of the spinal cord and brain seen here in the frog are common to all animals with backbones (vertebrates) - including humans.
Update: we can now confirm that the species is Litoria ewingii