Abb. 271: Stages of ontogenetic early development up to basic body form
(pharyngula) in amphibians (frog), birds (chicken) and humans. The
comparison (a-d) shows similarities but also clear
differences with regard to appearance and
course of development in the individual vertebrate classes. Even when
the typical vertebrate construction plan (pharyngula, d) is reached the
individual class pattern of features remains recognizable.