Pref 



ace 



A study of the comparative embryology of a group of animals such as the 

 vertebrates when followed to its logical conclusion leads to a consideration 

 of the comparative anatomy of the group. Students claim, and justly so, that 

 they learn best through the association of events, things, and concepts. As 

 applied to the study of vertebrate embryology and anatomy, the principle 

 of learning by association means this: observations upon the adult anatomy 

 of the various organ-systems of a particular vertebrate species when corre- 

 lated with the earlier stages of embryonic development of these systems lead 

 to a more ready perception and understanding of structural principles and 

 relationships involved. Furthermore, when the developmental anatomy and 

 the adult anatomy of any one species is associated with similar phenomena 

 in other species of the vertebrate group it naturally produces a clearer under- 

 standing of the development and morphology of the group as a whole. This 

 broad, comprehensive approach is a fundamental one and it is a requirement 

 for the furtherance of research in vertebrate biology, whether it be on the 

 level of cellular chemistry or the physiology of organ-systems. 



An endeavor to satisfy a demand for a comprehensive approach to verte- 

 brate development by an extension of the descriptions of the earlier phases 

 of the embryology of several representative vertebrate species into their final 

 stages of development, and hence into the realm of comparative anatomy, is 

 the main purpose of this book. This goal is the greatest defense which the 

 author can give for his effort to assemble the material and data contained 

 herein. 



On the other hand, though the book correlates comparative vertebrate em- 

 bryology with comparative vertebrate anatomy, its arrangement is such that 

 the fundamental features of comparative vertebrate embryology readily can 

 be divorced from the intricate phases of comparative anatomy. For example, 

 Chaps. 1-11, 20, 21, and 22 are devoted to a consideration of basic embryo- 

 logical principles whereas Chaps. 12-20 treat particularly the relationships of 

 comparative embryology and comparative anatomy. A proper selection of 

 descriptive material in Chaps. 12-20 (which may be done readily by a survey 

 of the outline heading each chapter) added to the basic embryological data 

 affords a basis for a thorough course in comparative vertebrate embryology. 



The selection of material suggested in the previous paragraph brings forth 

 another motive for writing this text. It has been the author's habit — one com- 



