

WOLFF'S KNOWLEDGE OF THE GERM-LAYERS. 239 



animals (Zoophytes), especially in Sponges, that the latter 

 retain their original simplicity. 



Finally, as a special proof of the prophetic genius of 

 Caspar Friedrich Wolff, due emphasis must be given to the 

 remarkable fact that that naturalist assumed the existence 

 of these four secondary germ-layers under the name of 

 " four systems formed on one type," the proof of which 

 was not furnished till half a century later by Baer. 77 

 (Cf. p. 46.) 



Eemak's three germ-layers. 



Outer, or 

 upper 

 layer 



Inner, or 

 under 

 layer 



The four secondary 

 germ-layers. 



The two primary 

 germ-layers. 



sory layer) |- Exoderm,"or skin- 



I layer, 



II. Middle germ- ( 2. Skin-fibrous layer ) 

 layer (motor-ger- j 3. Intestinal-fibrous \ 

 minative layer) I layer 



[ Vegetative layer, 

 | III. Inner, or under A Tm , M ,.. Ml , f Entoderm, or intes. 



Modified ontogenetic process. 



Original phylogenetic process 



