ABSTRACT
UNTIL fairly recently embryology has been in the main a comparative study. Its object has been conceived to be the derivation of a general scheme of development, of which the processes found in the different groups of animals could be regarded as modifications. In particular the diverse, but nevertheless obviously related, groups of vertebrates have provided fascinating material for study of the relations between different types of embryonic development. The wealth of available facts is indeed immense, and there are numerous textbooks devoted to it. Perhaps the best are Dalcq and Gerard's revision of Brachet's work and the recent book of Nelsen.
