2O GENERAL PRINCIPLES OF ZOOLOGY. 



aid of observation that the hen's egg at the beginning was- 

 without any organization, and that only gradually did the 

 various organs appear in it. In the embryo there takes 

 place a new formation of all parts, an Epigcncsis. This 

 first assault upon the evolutionist school was entirely with- 

 out result, chiefly for the reason that A. von Haller, the 

 most celebrated physiologist of the last century, used all 

 his influence to suppress the idea of Epigenesis. Wolff 

 was not able to establish himself in a circle of scientific 

 influence in Germany, and was obliged to emigrate to 

 Russia. OnJy after his death did his writings find, through 

 Oken and Meckel, proper recognition. 



Von Baer. Thus it remained for Carl Ernst von Baer 

 in his classic work, " Die Entwicklung des Hiihnchens, 

 Beobachtung und Reflexion " (1832), to establish Embry- 

 ology as an independent study. Baer confirmed Wolff's 

 doctrine of the appearance of layerlike rudiments, from 

 which the organs arose, and on account of the accuracy 

 with which he conducted the proof of this he is considered 

 the founder of the germ-layer tJicory. Further, he came 

 to the conclusion that each type had not only its peculiar 

 structural plan, but also its peculiar course of development ; 

 that for vertebrates an evolutio bigemina was character- 

 istic, for the articulates the evolutio gemina, for the mol- 

 lusks the evolutio contorta, and for the radiates the evo- 

 lutio radiata. Here we meet for the first time the idea 

 that for the correct solution of the conditions of relation- 

 ship of animals, and therewith results for a natural system- 

 ization, Comparative Embryology is indispensable; an idea 

 which in recent years has proved exceedingly fruitful. 



Cell Theory. Of fundamental importance for the fur- 

 ther building up of Comparative Anatomy and Embry- 

 ology was the proof that all organisms, as well as their 

 embryonic forms, were composed of the same elements, 

 the cells. This knowledge is the quintessence of the cell 

 theory, which during the first thirty years of this century 

 was propounded by Schleiden and Schwann, and which two- 

 decades later was completely reformed by the protoplasm 



