AUTHOR'S PRKFA< i: TO 'ini. iinsr i-:mTi< ix 



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

 in their 6880006 and their >i^ni!ieancc. Hut, thanks to (lie results of 

 Comparative Kmhryolou'y, lli" numher of the unintelligible ])rocesse8 

 has been every year (liininishrd, and in tin- same ratio the <t udy of 

 Kmbryol.i.iry e\en for the l.e^innei' has hem rendered ea>ier. 



At least, it is not in any way an es>ential feature of the pn 

 of development that it should he more difficult to understand than 

 the structure of the completed form, l-'m- every development begin* 

 with a very simple condition, from which the more complicated i> 

 gradually derived and hy which it is explained. 



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

 ology with especial interest, both in annually recurring academic 

 lectures and in a >eries of scientific investigations, the de-ire ha> 

 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 inr estimation in the foreground. I 

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

 tion 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. in>ti ad of being opposed to a comparative-morphological 

 direction, it can ho 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 ( 'hapter 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 *tate of a sci> ntilic question for the 

 time being has bt en attained. In pending controversial questions 



