baer's work. 53 



There, however, he forsook almost entirely his former field 

 of labour, and occupied himself with researches of a totally 

 different nature, in various branches of Natural Science, 

 especially in Geography, Geology, Ethnography, and Anthro- 

 pology. His works on the History of the Evolution of 

 Animals are far the most important ; nearly all of these 

 were completed while he was in Konigsberg, though some 

 of them were not published until later. Their merits, like 

 those of Wolff's writings, are many-sided, and extend over 

 the whole domain of Ontogeny in very various directions. 



Baer especially perfected the fundamental Theory of 

 Germ-layers, as a whole as well as in detail, so clearly and 

 completely, that his idea of it yet forms the safest basis of 

 our knowledge of Ontogeny. He showed that in Man and 

 the other Mammals, as in the Chick — in short, as in all Ver- 

 tebrates — first two, and then four germ-layers are formed, 

 always in the same manner, and that the modification of 

 these into tubes gives rise to the first fundamental organs 

 of the body. According to Baer, the first rudiment of the 

 body of a Vertebrate, as it appears on the globular yelk 

 of the fertilized egg, is an oblong disc, which first separates 

 into two leaves or layers. From the upper or animal layer 

 evolve all the organs which produce the phenomena of 

 animal life : the functions of sensation, of motion, and the 

 covering of the body. From the lower or vegetative layer 

 proceed all the organs which bring about the growth of the 

 body : the vital functions of nutrition, digestion, blood- 

 making, breathing, secretion, reproduction, and the like. 

 Each of these two original germ-layers separates again into 

 two thinner layers, or lamella3, one lying above the other. 

 First, the animal layer separates into two, which Baer calls 



