2 EARLY EMBRYOLOGY OF THE CHICK 



for those who are beginning the study of embryology and has 

 accordingly been kept as brief and as uncomplicated as possible. 

 Nevertheless it is assumed that the beginner in embryology 

 will not be without a certain back-ground of zoological 

 knowledge and training. He may reasonably be expected to 

 be familiar with some of the aspects of evolution and heredity, 

 with the recapitulation theory, the cell theory, the nature of 

 the various types of tissues, and the more general phases of 

 the morphology of vertebrates. Before laboratory work on 

 the chick is begun in any course in embryology the nature of 

 sexual reproduction, and the processes of gametogenesis, 

 maturation, fertilization and cleavage, will have been taken 

 up. It therefore seems unnecessary to include here any pre- 

 liminary, general discussion of these phenomena. References 

 for collateral reading on this and other phases of the subject 

 are given in the appendix. 



Like other sciences embryology demands first of all accurate 

 observation. It differs considerably, however, from such a 

 science as adult anatomy where the objects studied are rela- 

 tively constant and their component parts are not subject to 

 rapid changes in their inter-relations. During development, 

 structural conditions within the embryo are constantly chang- 

 ing. Each phase of development presents a new complex of 

 conditions and new problems. 



Solution of the problems presented in any given stage of 

 development depends upon a knowledge of the stages which 

 precede it. To comprehend the embryology of an organism 

 one must, therefore, start at the beginning of its development 

 and follow in their natural order the changes which occur. 

 At the outset of his work the student must realize that proper 

 sequence of study is essential and may not be disregarded. A 

 knowledge of structural conditions in earlier stages than that 

 at the moment under consideration, and an appreciation of the 

 trend of the developmental processes by which conditions at 

 one stage become transmuted into different conditions in the 

 next, are direct and necessary factors in acquiring a real com- 

 prehension of the subject. Without them the story of 

 embryology becomes incoherent, a mere jumble of confused 

 impressions. 



A knowledge of the phenomena of development is ordinarily 



