tKTBODl ' 3 



Amphibia. , etc, only throuirh tl btH -li various 



object* has insight bern acquired into many processes, which in tln-ir 



Q06 remained unintelligible to n* from I h- st inly of th<- i 

 alone. 1-or it was thus tliat on.- tir-t learned to diMingiiUli the 

 important from tin- fc . nd unimportant, and to iind.-i | 



tin- laws of development in their generalitv. 



In this text Look, therefor.-. I shall n..t confine myself bo single 

 object, surh as the egg of tin- Hen or tin- Hal. hit, Inif from more 

 LTt'nrral foinpai-ative standpoints shall riidra vuiir to pn-MMit P 

 through rxtiMi.-sivi- >-rii-s of in\ --t iirat ions, \vc ha\.> t hus far recognised 

 as tiif rule in n irard to the real natiu'c of the processes of f'i-tili>:i- 

 tion and clcaxaut'. tin- formation of the germ layers, etc. 



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

 loirv. The purpose and theprohlem is lirst of all to learn to com- 

 prehend thi' development and the structure of the human l>ody. 

 \\hatwe know about that has been placed before everything else, 

 and the einhryoloiry of the remaining Vertebrates has been c; 



\\ere. fully utilised, only in so far as was necv 

 purpo>e indicated. 



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

 cording to the separate >\>hnis <if ortrans, thero is n long Series ol 

 processes, with which tin 1 development begms, wlnoil do not. permit 

 of an arrangement, because at the beginning the: fundaments of 

 definite, afterwards dillerentiated organs, are not recognisahle it 

 ir'rm. Before there is any formation of organs the eiru' is di 

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

 larger complexes, which have been calle 1 the germ-layers, or the 

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

 brates there are formed certain orirms which are useful only d'. 

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

 hranes and tn-tal appendages. All of the processes of th 

 \s.' >liall mat of connectedly, and by them>elves. In aec..:-dance 

 with this, we can divide our theme into two main section^. 

 of which will deal with the initial p: ;' development and the 



emhrvonic membrane-, the cond \\illi ;)ie oiiirin of the separate 



ma of organs, In order to facilitate for the advanced a more 



thorough study, and a penetration into embryologum! literal ure, a 

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

 the separate chapters. On the other hand, b t of Kml 



logy may be mentioned in this place. [Compare al>o the larger 

 monographic works cited at the end of the book.] 



