EDITOR'S FOREWORD 



Modern biological science is becoming more and more concerned with 

 mechanism and function. Years ago the biological research worker and the 

 teacher of biology were content to describe in infinite detail the vast com- 

 plexities of animal and plant structure. For such study only one technique 

 was necessary. The beginning student of biology — and the advanced student 

 as well — was taught only to look, see, draw. As a result many a young in- 

 quiring mind became bored with the endless detail and the exasperating 

 terminology of biological science and turned toward other sciences that 

 offered a greater challenge to thought and imagination. As a matter of fact, 

 in such sciences as physics and chemistry, the trend both in research and 

 teaching has been toward inquiry into ultimate mechanism. Impressed with 

 this general trend, the present-day student of biology is no longer con- 

 tent with drawing and memorizing details. He wants to know the why and 

 the wherefore. 



Modern teaching in various branches of biology tends more and more 

 toward such a point of view. Invertebrate zoology is becoming physiological, 

 genetics is searching for an explanation of the gene, a recent text on cytology 

 is essentially a cell physiology. In the field of embryology a strange situation 

 exists. Present-day research is largely theoretical and experimental. Men who 

 do research in embryology are not happy with the essentially descriptive 

 textbooks they have had to use in their teaching. Undoubtedly the present 

 book meets a deep and sincere demand. 



Years ago the science of embryology was nothing more than a detailed 

 account of the infinite succession of stages that intervened between the egg 

 and the adult. These stages are in themselves interesting, but it is doubtful 

 whether the mind of a beginning student should be burdened with too many 



