Ontogeny of Embryonic Behavior in Aves. XII. Stages in the Development of Physiological Activities in the Chick Embryo

Abstract

This paper proposes (1) to summarize in a convenient and readable form the mass of data concerning the physiological activities of the chick embryo which has accumulated during several years from observations on several thousand eggs, (2) to revise from new material certain statements made in previous papers of this series, and (3) to attempt a division into developmental stages of physiological activity during embryonic weeks. This division should serve as a useful reference for future experimental studies of the physiology and behavior of avian embryos at large and also for class-instruction in these subjects. Like morphological development, the development of physiological and behavioral activities is gradual and continuous. The proposed subdivision into stages is made for the sake of convenience and exposition. Since individual variations, both morphological and physiological, are very great during the first four days of incubation, the early age of the embryo will be designated by the number of pairs of somites rather than by the period of incubation. For normal embryos in normal position, however, extreme variations in physiological and behavioral functions are fairly rare after five days of incubation. It seems safe, therefore, to describe embryonic age after this time in terms of incubation-days. As abnormal structure and abnormal embryonic positions or abnormal environmental conditions in the egg will almost always produce abnormal behavior, our description will be confined to the normal. A summary of morphological development in the chick. While the early morphological development of the chick has been described at length in many text-books of embryology, the following brief sketch should help the reader to appreciate the significance of the physiological activities which correspond to anatomical conditions at each stage.