An essential requisite to the understanding of anatomy, either human or comparative, is a thorough appreciation of the facts and processes of embryology. This subject has been taught by laboratory methods in our colleges of liberal arts and sciences for only three decades, and in our medical schools, with a very few exceptions, the laboratory teaching of embryology has been introduced within the last decade. Owing both to the availability of material and also to the fact that the text-book of Foster and Balfour was, for two decades, the only book available in embryology, the greater part of the laboratory teaching of embryology, until very recent years, has been on the chick, and therefore this form has come to be a classic. Now, however, especially in the medical schools, mammalian material is more and more used to illustrate the facts of embryology. For certain phases of development, however, the chick