232 THE EVOLUTION OF MAN. 



say, the low Plant-animals (Sponges, simplest Polyps, etc.), 

 remain throughout their life stationary in a structural stage 

 which differs very little from the Gastrula"; their whole body 

 being composed of only two cell-strata or layers. This fact is 

 of the very greatest significance. For we see that Man, and 

 indeed all Vertebrates, pass quickly through a transitory 

 two-layered structural stage, which is persistently retained 

 throughout life by these lowest Plant-animals. By now 

 again applying our first principle of Biogeny, we im- 

 mediately obtain the following very important conclusion : 

 Man and all those other animals, which in the first stages of 

 their individual evolution pass through a two-layered struc- 

 tural stage or a Gastrula-form, must have descended from a 

 primceval, siryiple parent-form, the tvhole body of %vhich 

 consisted throughout life, as noiu in the case of the lowest 

 Plant-animals, only of tiuo different cell-strata or germ- 

 layers. To this most important primaeval parent-form, to 

 which we shall presently refer in detail, we will now pro- 

 visionally give the name of the Gastrsea (i.e. primitive intes- 

 tinal animal).^* 



According to the Gastrsea theory, there is in all animals 

 one organ which is originally of the same morphological 

 and physiological significance ; this is the primitive intes- 

 tine ; the two primary germ-layers, which form the wall of 

 this intestine, must therefore in all cases be regarded as also 

 of the same significance, or as " homologous." This import- 

 ant " homology of the two primary germ-layers " is, on the 

 one hand, demonstrated by the fact that the Gastrula in all 

 cases originates in one way, that is, by the inversion (in- 

 vagination) of the Blastula ; and, on the other hand, by the 

 fact that in all cases the same fundamental organs arise 



