But Haeckel's drawings are wrong. Photographing actual embryos at these stages, Richardson and colleagues show that Haeckel's drawings are oversimplified to the point of obscuring important differences between classes of vertebrates. The Richardson et al. paper does not dispute that there is a highly conserved embryonic stage among the vertebrate classes. However, these authors do criticise the notion that this stage is nearly identical in all species and that differences beteen the classes can be resolved only after subsequent development. Rather, they discover significant differences between groups.
Size is one distinctive marker.
The scorpion fish embryo is 700 microns long at the tailbud stage, while the mudpuppy salamander measures some 9 millimeters. Heterochrony is another problem. In some species of direct developing frogs, and in monotreme mammals, limb buds are already present at the tailbud stage, whereas in other species, these are not seen until significantly later. Birds are characterized by their prominent mesencephalon. Whereas most amniote embryos have a heart by this stage, the zebrafish does not.