Wolff's knowledge of the gekm-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.) 



Remak's three germ-layers. 



Outer, or 

 upper 

 layer 



Inner, or 

 under 

 layer 



I. Outer, or upper 

 germ-layer (sen- 

 sory layer) 



II. Middle germ- 

 layer (motor-ger- 

 minative layer) 



III. Inner, or under 

 germ-layer (tro- 

 phic layer) 



The four secondary 

 germ-layer$. 



1. Skin-sensory 

 layer 



1 2. Skin -fibrous layer 

 '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 phyiogenetic process. 



