INTRODUCTION 3 



acquired by studying a series of embryos at various stages of 

 advancement. Each stage should be studied not so much for 

 itself, as for the evidence it affords of the progress of develop- 

 ment. In the study of embryology it does not suffice to acquire 

 merely a series of "still pictures" of various structures, however 

 accurate these pictures may be. The study demands a constant 

 application of correlative reasoning and an appreciation of the 

 mechanical factors involved in the relations of various structures 

 within the embryo to each other, and in the relation of the 

 embryo as a whole to its environment. In order to really 

 comprehend the embryological significance of a structure one 

 must know not only its relations within the embryo being 

 studied at the time, but also the manner in which it has been 

 derived and the nature of the changes by which it is progressing 

 toward adult conditions. To get absolutely the whole story it is 

 obvious that one would have to study a series of embryos with 

 infinitely small intervals between them. Nevertheless the 

 fundamental steps in the process may be grasped from a much 

 less extensive series. The fewer the stages studied, however, 

 the more careful must one be to keep in mind the continuity 

 of the processes and to think out the changes by which one stage 

 leads to the next. 



The outstanding idea to be kept in mind by the student begin- 

 ning the study of embryology is that the development of an 

 individual is a process and that this process is continuous. The 

 conditions he sees in embryos of various stages are of importance 

 chiefly because they serve as evidence of events in the process 

 of development at various intervals in its continuity, as his- 

 torical events are evidences of the progress of a nation. Just 

 as historical events are led up to by preparatory occurrences and 

 followed by results which in turn affect later events, so in em- 

 bryology events in development are presaged by preliminary 

 changes and when consummated affect in turn later steps in 

 the process. 



In certain respects the laboratory study of embryological 

 material involves methods of work for which courses in general 

 zoology do not entirely prepare the student. Some general 

 suggestions as to methods of procedure are, therefore, not out 

 of place. 



In dissecting gross material it is not unduly difficult to 



