AUTHOR'S PREFACE TO THE FIRST EDITION. ix 



take into account, and when many processes were not yet understood 

 in their essence and their significance. But, thanks to the results of 

 Comparative Embryology, the number of the unintelligible processes 

 has been every year diminished, and in the same ratio the study of 

 Embryology even for the beginner has been rendered easier. 



At least, it is not in any way an essential feature of the process 

 of development that it should be more difficult to understand than 

 the structure of the completed form. For every development begins 

 with a very simple condition, from which the more complicated is 

 gradually derived and by which it is explained. 



Inasmuch as I have for twelve years pursued the study of Embry- 

 ology with especial interest, both in annually recurring academic 

 lectures and in a series of scientific investigations, the desire has 

 been awakened in me to acquire for Embryology a broader and more 

 secure foundation in education, and to procure for it admission into 

 larger circles of medical men and well-educated naturalists. As the 

 result of this there has come into existence the book which is before 

 us, in which the especial problem has been to make the complicated 

 structure of the human body more intelligible through the knowledge 

 of its development. 



For the solution of this problem I have in the present text-book 

 placed the comparative method of investigation in the foreground. I 

 do not thereby find myself in any way in opposition to another 

 direction of embryological research, which places the objective point 

 in the physiological or mechanical explanation of the form of the 

 animal body. Such a direction I hold to be fully warranted, and I 

 believe that, instead of being opposed to a comparative-morphological 

 direction, it can be of the most permanent value to it in the solution 

 of its problems. One will find that I have here given full attention 

 to the mechanico-physiological explanation of forms. Compare the 

 sections on cell-division and Chapter IV., " General Discussion of the 

 Principles of Development," in which the laws of unlike growth and 

 the processes of the formation of folds and evaginations are treated. 



In the presentation of the separate processes of development, in 

 the main the important things only have been selected, the sub- 

 sidiary left out, in order thus to make the introduction into 

 embryological study easier. In the case of fundamental theories 

 I have gone into their history extensively, because it is of great 

 interest, and under certain circumstances operates as a stimulus, 

 for one to see in what way the state of a scientific question for the 

 time being has been attained. In pending controversial questions 



