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.''^'^ 

 (C£ p. 46.) 



Remalc's three germ-lay ei s. 



Cater, or ( I. Outer, or upper 

 upper I germ-layer (sen- 

 layer [ sory layer) 



Inner, or 

 under -! 

 layer 



^ II. Middle germ, 

 layer (motor-ger- / 

 minative layer) 



III. Inner, or under 

 germ-layer (tro- 

 phic layer) 



The four secondary 

 germ-layers. 



1. Skin -sensory 



layer ^ 



2. Skin-fibrouslayerJ 



3. Intestinal-fibrous ^ 

 layer 



. Intestinal-glan- 

 dular layer 



The two primary 

 germ-layers. 



Animal layer, 

 Exoderm, or skin- 

 layer. 



Vegetative layer, 

 ^ Entoderm, or intes- 

 tinal layer. 



Modified ontogenetic process. Original phylogenetic process. 



