CHAPTER 1 

 INTRODUCTION 



The only method of attaining a comprehensive understanding 

 of embryological processes is through the study and comparison 

 of development in various animals. Many phases of the 

 development of any specific organism can be interpreted only 

 through a knowledge of corresponding processes in other 

 organisms. The beginning student, however, must acquire his 

 knowledge of embryology through intensive study of one form 

 at a time, depending at first on older workers in the field for 

 interpretation of the phenomena encountered. Building on 

 the familiarity with fundamental processes of development thus 

 acquired, he may later broaden his horizon by the comparative 

 study of a variety of forms. 



The chick is one of the most satisfactory animals on which 

 student laboratory work in embryology may be based. Chick 

 embryos in a proper state of preservation and of the stages 

 desired can be readily secured and prepared for study. Used 

 as the only laboratory material in a brief course they afford a 

 basis for understanding the early differentiation of the organ 

 systems and the fundamental processes of body formation 

 common to all groups of vertebrates. In more extended courses 

 where several forms are taken up, the chick serves at once as a 

 type for the development characteristic of the large-yolked 

 eggs of birds and reptiles, and as an intermediate form bridging 

 the gap between the simpler processes of development in fishes 

 and amphibia on the one hand and the more complex processes 

 in mammals on the other. In medical school courses where a 

 knowledge of human embryology is the end in view the chick 

 not only makes a good stepping stone to the understanding of 

 mammalian embryology, but also provides material for the 

 study of early developmental processes not readily demon- 

 strable in mammalian material. 



This book on the development of the chick has been written 



