INTRODUCTION. 5 



as being arrangements of these particles or cells variously 

 modified, and that all the phenomena of the form and 

 structure of living beings were to be regarded as the results 

 of a variable nutritive energy, to which he gave the name 

 vis essentialis. 



Haller complained of Wolff, that he had attempted to 

 make a great leap instead of being contented with small on- 

 ward steps. Wolff's leap proved too great for his time. While 

 his insight into the fundamental doctrines of histology re- 

 mained for the most part without fruit till the next century, 

 so also the way he opened up in embryology was successfully 

 followed by no one for many years after. 



In 1816 that admirable teacher Dollinger, of Wiirzburg, 

 induced Pander to take up the study of the incubated hen's 

 egg. We owe to Pander (Dissertatio Inauguralis sistens 

 Historiam Metamorphoseos quam Ovum Incubatum prioribus 

 quinque diebus subit, and Beitrdge zur Entwickelungsge- 

 schichte des Huhnchens im Eie) a clear and excellent descrip- 

 tion of many of the changes which take place during the 

 early days of incubation. It was he who introduced the term 

 blastoderm. He too first drew attention to the distinction of 

 the three layers, serous, mucous, and vascular. But his 

 greatest merit perhaps consisted in the fact of his studies 

 having been the exciting cause of those of Von Baer. 



Coming to Wiirzburg to study under Dollinger, and finding 

 Pander busily engaged in his embryological work, Von Baer 

 enthusiastically took up the same subject, and thenceforward 

 devoted the greater part of his life to it. 



Of the results of his labours, which are embodied in his 

 Entwickelungsgeschichte der Thiere, 1828, 1837, this simply 

 may be said. Von Baer found the true line of inquiry already 

 marked out by Wolff. He followed up that line so sedulously 

 and with such success, that nearly all the work which has 

 been done since his day up to the present time, in Vertebrate 



