196 THE EVOLUTION OF MAN. 



between the two germ-layers ; this cavity, filled with blood 

 or lymph, is the body-cavity (cceloma).^ 



The two primary germ-layers, the outer or serous, and 

 the inner or mucous layer, were first clearly distinguished, 

 in 1817, by Pander, in the incubated Chick (p. 51). But 

 their full significance was first thoroughly recognized by 

 Baer, who, in his "History of Evolution" (1828), gave the 

 name of animal layer to the outer layer, that of vegetative 

 layer to the inner. These names are very apt, because it is 

 the outer layer which especially (if not exclusively) gives rise 

 to the animal organs of sensation and movement, the skin, 

 the nerves, and the muscles ; while, on the other hand, it is 

 especially from the inner layer that the vegetative organs 

 of nourishment and reproduction, the intestine and blood- 

 vessel system in particular, arise. Twenty years after- 

 wards (in 1849) Huxley pointed out that in many low 

 Plant-animals (Zoopkyta), such as the Medusae, the whole 

 body permanently consists only of these two primary 

 germ-layers. The outer of these he called the ectoderm, or 

 exoderm ; the inner he named the endoderm, or entoderm. 

 Recently Kowalevsky and Ray Lankester especially have 

 tried to show that other Invertebrate animals of the 

 most diverse classes, in Worms, Soft-bodied Animals (Mol- 

 lusca}, Star-animals (Echinodermci), and Articulated animals 

 (Arthropoda), form from the same two primary germ- 

 layers. Lastly, I have myself shown that this is the case 

 also in the lowest Plant-animals, in Sponges ; and at the 

 same time I tried to prove in my Gastrsea Theory that these 

 two primary germ-layers must be considered as of the same 

 significance, or as homologous, in all cases, from Sponges 

 and Corals to Insects and Vertebrates, including Man. 



