AUTHOR'S PREFACE 



TO THE THIRD EDITION. 



IN the two yeais which have elapsed since the appearance of the 

 second edition of this text-book, our knowledge of the embryology of 

 Vertebrates has experienced many important enrichments, thanks to 

 the numerous investigations which are annually published. There- 

 fore, as the problem of preparing a third edition of the text-book 

 confronted me, I was compelled to make extensive changes in 

 many places. Thus the second and third chapters, concerning the 

 processes of fertilisation and cleavage of the egg, have undergone 

 expansion, owing to the presentation of the important discoveries 

 which have been made on the the egg of Ascaris megalocephala. I 

 have given an entirely new wording to the ninth chapter on the 

 development of connective substance and blood, also to the 

 sections on the origin of the urinary organs and the development of 

 the peripheral nervous system, and, finally, to the account of the 

 development of the heart and the venous system. Also at other 

 places one will often recognise the hand of improvement. 



The third edition has been essentially improved by the addition of 

 thirty new figures, which I have taken from the investigations of 

 VAN BENEDEN, BOVERI, DUVAL, FLEJIMING, HERMANN, His, BORN, 

 GEGENBAUR, NAGET,, VAN WIJHE, GRAF SPEE, BONNET, and KEIBEL. 

 Through the friendliness of Professor VAN BENEDEN I was also put 

 in a position to employ for my text-book three figures out of his 

 hitherto unpublished extensive work on the development of the 

 germinal layers of the Rabbit. By means of the increase in the 

 number of figures I hope that I have been able to render still easier 

 the comprehension of many of the processes of development. 



And so I close the preface to the third edition by expressing 

 my thanks to all those who have rendered me friendly aid, and 

 especially to the publisher, who in the further equipment of the 

 text-book has met my wishes with the greatest willingness. 



OSCAR HERTWIG. 

 BERLIN, March 1890. 



