WOLFFS 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.''^ 

 (Cf. p. 46.) 



Remak's three germ-layefi s. 



Outer, or ( I. Outei-, or upper 

 upper I germ-layer (sen- 

 layer I sory layer) 



Inner, or 

 nnder -^ 

 layer 



'' II. Middle germ.i 

 layer (motor-ger- ^ 

 niinative layer) 



III. Inner, or nnder 

 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, orintefci' 

 tinal layer. 



Modified ontogenetic process. J Original pliylogenetic process. 



