^E 



PREFACE 



The fact that most courses in vertebrate embryology deal to 

 a greater or lesser extent with the chick seems to warrant the 

 treatment of its development in a book designed primarily 

 for the beginning student. To a student beginning the study 

 of embryology the very abundance of information available in 

 the literature of the subject is confusing and discouraging. He 

 is unable to cull the essentials and fit them together in their 

 proper relationships and is likely to become hopelessly lost in a 

 maze of details. This book was written in an effort to set forth 

 for him in brief and simple form the early embryology of the 

 chick. It does not purport to treat the subject from the com- 

 parative view point, nor to be a reference work. If it helps the 

 student to grasp the structure of the embryos, and the sequence 

 and significance of the processes he encounters in his work on the 

 chick, and thereby conserves the time of the instructor for inter- 

 pretation of the broader principles of embryology it will have 

 served the purpose for which it was written. 



In preparing the text, details have been largely omitted and 

 controverted points avoided for the sake of clarity in outlining 

 fundamental processes. While I would gladly have avoided 

 the matters of cleavage and germ layer formation in birds, a 

 brief description of them seemed necessary. Without some 

 interpretation of the initial phases of development, the student 

 has no logical basis for his study of the already considerably 

 developed embryos with which his laboratory work begins. 

 The treatment which it is desirable to accord to gametogenesis 

 and maturation as processes leading toward fertilization would 

 vary so greatly in extent and view point in different courses 

 that it seemed inadvisable to attempt any general discussion 

 of these phenomena. 



The account of development has not been carried beyond the 

 first four days of incubation. In this period the body of the 

 embryo is laid down and the organ systems are established. 

 Courses in general embryology rarely carry work on the chick 

 beyond this phase of development. More extensive courses in 



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