HOMOLOGY OF THE GERM-LAYERS. 233 



from the two gei'm-layers. The outer or animal germ-layer, 

 the skin-layer, or exoderm, always forms the outer body- 

 wall with the most important organs of animal life ; the 

 skin-covering, nerve-system, organs of the senses, etc. On 

 the other hand, the inner or vegetative germ-layer, the in- 

 testinal layer, or entoderm, gives rise to the inner intestinal 

 wall with the most important organs of vegetative life ; the 

 organs of nutrition, of digestion, those which form the blood, 

 etc. 



In these low Plant-animals, especially in Sponges, 

 the whole body of which remains permanently stationary 

 in the same structural stage, these two functional groups 

 (the animal and the vegetative acts) also continue strictly 

 distributed between the two simple, primary germ-layers. 

 Throughout life the outer or animal germ-layer retains the 

 simple significance of a covering (an outer skin), and, at 

 the same time, accomplishes the movements and sensations 

 of the body. On the other hand, the inner cell-stratum, 

 or the vegetative germ-layer, always retains the simple 

 significance of an intestinal epithelium, a nutritive in- 

 testinal cell-stratum, and in addition to this appears only 

 to produce the reproductive cells.^ 



In all other animals, and especially in all Vertebrates, 

 the Gastrula appears only as a very transitory germ-stage. 

 The two-layered stage of their germ-rudiment changes 

 quickly, first into a three-layered, and then into a four- 

 layered stage. On the completion of the germ-layers, which 

 lie one over the other, we have again provisionally attained 

 a fixed and definite point of view ; and one from which we 

 may trace and explain the incidents in the construction, 

 which are much more obscure and intricate. Trustwortliy 



