OUR EMBRYONIC DEVELOPMENT. 57 



were created together." Wolff, who had to go to St. 

 Petersburg, was long in his grave before the forgotten 

 facts he had observed were discovered afresh by Oken 

 at Jena in 1806. 



After Wolff's " epigenesis theory " had been estab- 

 lished by Oken and Neckel (whose important work 

 on the development of the alimentary canal was 

 translated from Latin into German), a number of 

 young German scientists devoted themselves eagerly 

 to more accurate embryological research. The most 

 important and successful of these was Carl Ernst Baer. 

 His principal work appeared in 1828, with the title, 

 History of the Development of Animals : Observations 

 and Reflections. Not only are the phenomena of 

 the formation of the germ clearly illustrated and 

 fully described in it, but it adds a number of very 

 pregnant speculations. In particular, the form of 

 the embryo of man and the mammals is correctly 

 presented, and the vastly different development of 

 the lower invertebrate animals is also considered. 

 The two leaf-like layers which appear in the round 

 germ-disk of the higher vertebrates first divide, 

 according to Baer, into two further layers, and these 

 four germinal layers are transformed into four tubes, 

 which represent the fundamental organs — the skin- 

 layer, the muscular-layer, the vascular-layer, and the 

 mucous-layer. Then, by very complicated evolutionary 

 processes, the later organs arise, in substantially 

 the same manner in man and all the other vertebrates. 

 The three chief groups of invertebrates, which in 

 their turn differ widely from each other, have a very 

 different development. 



One of the most important of Baer's many dis- 

 coveries was the finding of the human ovum. Up to 



