HISTORY OF ZOOLOGY. 17 



spermatozoon must be the young creature wliicli found favorable 

 conditions for growth in the store of food in tlie egg, or the egg 

 represents tlie individual and was stimulated to the ' evolutio ' by 

 the spermatozoon. This theory led to the doctrine of inclusion, 

 which taught that in the ovary of Eve were included the germs of 

 all human beings who have lived or ever will live. 



Caspar Friedrich WolfE combated this idea with his " Theoria 

 generationis " (17511); he sought to prove by observation that the 

 hen's egg at the beginning is without any organization, and that 

 gradually the various organs appear in it. In the embryo there is 

 a new formation of all parts, an Epigenesis. This first assault 

 upon the evolutionist school was entirely without result, chiefly 

 because Albrecht von llaller, the most celebrated physiologist of 

 the eighteenth century, by his influence sujojjressed the idea of 

 epigenesis. Wolff was not able to establish himself in scientific 

 circles in Germany, and was oVjliged to emigrate to Russia. Only 

 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 embryology as an independent 

 study. Baer confirmed Wolff's doctrine of the appearance of 

 layerlike Anlagen, from which the organs arose; and on account 

 of the accuracy with which he proved this he is considered the 

 founder of the germ-layer theory. Further, he came to the con- 

 clusion that each type had not only its peculiar structural plan, 

 but also its j)eculiar course of development; that for vertebrates 

 an evolutio bigemina was characteristic, for the articulates the 

 evolutio gemina, for the molluscs the evolutio contorta, and for 

 the radiates the evolutio radiata. Here we meet for the first time 

 the idea that for the correct solution of the questions of relation- 

 ship of animals, and therefore a basis for a natural classification, 

 comjDarative embryology is indispensable; an idea which in recent 

 years has proved exceedingly fruitfiil. 



Cell Theory. — Of fundamental importance for the further 

 growth of comparative anatomy and embryology was the j^roof 

 that all organisms, as well as their embryonic forms, were com- 

 posed of the same elements, the cells. This knowledge is the 

 quintessence of the cell theory, which during the third decade of 

 the last century was advanced by Schleiden and Schwann, and 

 which two decades later was completely remodelled by the proto- 

 plasm theory of Max Schultze. In the cell theory a simjole prin- 



