12 GENERAL PRINCIPLES OF ZOOLOGY 



pcndently side by side, within which again there are higher and lower 

 forms. The position of an animal is determined by two factors: first, by 

 its conformity to a type, by the structural plan which it represents; second, 

 by its degree of organization, by the stage to which it attains within its type. 



Comparative Embryology. Evolution vs. Epigenesis. The 

 same results which Cuvier reached by the way of comparative anatomy 

 were attained two decades later by C. E. von Baer by the aid of embry- 

 ology. Embryology is the youngest branch of zoology. The difficulties 

 of observation, due to the delicacy and the minuteness of the develop- 

 mental stages, were lessened by the invention of the microscope and 

 microscopical technique. Further, there was no idea of developmental 

 history in the present sense of the word ; each organism was thought to be 

 laid down from the first complete in all its parts, and only needed growth 

 to unfold its organs (evolutid) ; either the spermatozoon must be the young 

 creature which found favorable conditions for growth in the store of food 

 in the egg, or the egg represents the individual and was stimulated to the 

 'evolutio' by the spermatozoon. This theory led to that 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 Wolff combated this idea (1759) ; he sought to prove 

 by observation that the hen's egg at the beginning is without organization, 

 and that gradually the various organs appear in it. In the embryo there 

 is a new formation of all parts, an epigenesis. This assault upon the 

 evolutionist school was without result, chiefly because Albrecht von 

 Haller, the most celebrated physiologist of the eighteenth century, sup- 

 pressed the idea of epigenesis. 



Von Baer. Carl Ernst von Baer in his classic work, "Die Entwick- 

 lung des Hlihnchens, Beobachtung und Reflexion" (1832), established 

 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 conclusion that 

 each type had not only its peculiar structural plan, but also its peculiar 

 course of development. Here we meet for the first time the idea that for 

 the solution of the questions of relationship of animals, and therefore a 

 basis for a natural classification, comparative embryology is indispensable; 

 an idea which in recent years has proved exceedingly fruitful. 



Cell Theory. Of fundamental importance for the further growth of 

 comparative anatomy and embryology was the proof that all organisms, 

 as well as their embryonic forms, were composed of the same elements, 

 the cells. This cell theory, was advanced by Schleiden and Schwann 



