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 

 (Of. P . 46.) 



Remak's three germ-layei s. 



Outer, or 

 upper 

 layer 



Inner, or 

 tinder 

 layer 



II. Outer, 

 germ-la 

 sory la; 



or upper 

 layer (sen- 

 sory layer) 



II. Middle germ- 

 lay er ( inotor-ger- 

 minative layer) 



I III. Inner, or tinder 

 germ-layer (tro- 

 phic layer) 



The four secondary 

 germ-layers, 



1. Skin. sensory 

 layer 



2. Skin-fibrous layer 



3. Intestinal-fibrous 



layer 



4. 



Intestinal -glan- 

 dular layer 



The two primary 

 germ-layers. 



Animal layer, 

 Exoderm, or skin 

 layer. 



Vegetative layer, 

 Entoderm, or intes- 

 tinal layer. 



Modified ontogenctic process. 



Original phylogenetic process. 



