INTRODUCTION. 3 



Amphibia, Fishes, etc. Only through the observation of such various 

 objects has insight been acquired into many processes, which in their 

 essence remained unintelligible to us from the study of the Chick 

 alone. For it was thus that one first learned to distinguish the 

 important from the accessory and unimportant, and to understand 

 the laws of development in their generality. 



In this text-book, therefore, I shall not confine myself to a single 

 object, such as the egg of the Hen or the Rabbit, but from more 

 general comparative standpoints shall endeavour to present what, 

 through extensive series of investigations, we have thus far recognised 

 as the rule in regard to the real nature of the processes of fertilisa- 

 tion and cleavage, the formation of the germ layers, etc. 



However, let no one expect a text-book of comparative Embryo- 

 logy. The purpose and the problem is first of all to learn to com- 

 prehend the development and the structure of the human body. 

 What we know about that has been placed before everything else, 

 and the embryology of the remaining Vertebrates has been cited, and, 

 as it were, fully utilised, only in so far as was necessary for the 

 purpose indicated. 



In the division of the embryological material proposed by us, ac- 

 cording to the separate systems of organs, there is a long series of 

 processes, with which the development begins, which do not permit 

 of an arrangement, because at the beginning the fundaments of 

 definite, afterwards differentiated organs, are not recognisable in the 

 germ. Before there is any formation of organs, the egg is divided 

 into numerous cells, and these then arrange themselves into a few 

 larger complexes, which have been called the germ-layers, or the 

 primitive organs of the embryo. Further, in the higher Verte- 

 brates there are formed certain organs, which are useful only during 

 embryonic life, and are subsequently lost namely, the foetal mem- 

 branes and fcetal appendages. All of the processes of that nature 

 we shall treat of connectedly, and by themselves. In accordance 

 with this, we can divide our theme into two main sections, the first 

 of which will deal with the initial processes of development and the 

 embryonic membranes, the second with the origin of the separate 

 systems of organs. In order to facilitate for the advanced a more 

 thorough study, and a penetration into embryological literature, a 

 survey of the more important original works is given at the close of 

 the separate chapters. On the other hand, text-books of Embryo- 

 logy may be mentioned in this place. [Compare also the larger 

 monographic works cited at the end of the book.] 



