Micro photographs are used to show further stages of gestation of the ovum. Whilst in the interior a mesoderm is undergoing differentiation and the medullary plate is forming on the surface. At the caudal end of the medullary plate (left half in pictures) the primitive orifice is visible. The two folds of the medullary place grow over to meet each other and finally coalesce, thus forming the medullary tube. There is a time lapse shot of this development. Further differentiation in the embryo takes place during a rotary movement which is shown greatly accelerated on the film. Time lapse shows accelerated rotation of the ovum (seen spinning).