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.''^ 

 (Cf. p. 46.) 



Remak's three germ-layei s. 



The four secondary 

 germ-lay ei's. 



Cater, or ( I. Outer, or npper , ^, . 



1 / 1. Skm. sensory 



eerm-laver (sen- , •' 



layer 



upper 

 layer 



Inner, or 

 under 

 layer 



germ-layer (sen- 

 sory layer) 



II. Middle germ- 1 2. Skin-fibrons layer. 



layer(motor-ger-|3. Intestinal-fibrous ^ 

 minative layer) I layer 



III. Inner, or under . t i i- i i 



, ,. 4. Intestmal-glan- 



germ-layer (tro- " 



The two primary 

 germ-layers. 



Animal layer, 

 ■ Exoderm, or skin- 

 layer. 



phic layer) 



dular layer 



Vegetative layer, 

 Entoderm, or intes- 

 tinal layer. 



Modified ontogenetic process. 



Original phylogenetic process. 



