HUXLEY AND KOWALEVSKY. C>'] 



difficulty of the very complex task which lies before this 

 science, and partly by the insufficient general preparation 

 possessed by most of the more recent students. 



All valuable modern investigations into Animal Onto- 

 genesis have only tended to confirm and add to the Theory 

 of Germ-layers as established by Baer and Remak. As the 

 most important advance made in this direction, it is deserv- 

 ing of mention, that the same two primary germ-layers, 

 from which the body of Vertebrates, including Man, 

 develops, have recently been shown to exist in all inver- 

 tebrate animals also, with the single exception of the lowest 

 group, that of the Primaeval animals {Protozoa.) The dis- 

 tinguished English naturahst, Huxley, in the year 1849, 

 had already shown that this is also true of Plant-animals 

 {Medusae). He drew attention to the fact that the two 

 cell-layers, from which the body of this Plant-animal 

 develops, correspond, morphologically as well as physio- 

 logically, to the two primary germ-layers of Vertebrates. 

 The upper germ -layer, from which the outer skin and the 

 flesh proceed, he named Ectoderm, or Outer layer ; the 

 lower, which forms the organs of digestion and reproduc- 

 tion, he called the Entoderm, or Inner layer. But during 

 the past ten years, the two germ-la^^ers have been found to 

 exist among many other Invertebrates. The indefatigable 

 Russian zoologist, Kowalevsky, has found them among 

 widely differing groups of Invertebrates, in Worms, 

 Star- animals {Echinoderma), Soft-bpdied animals {Mollusca), 

 Ai'ticulates {Arthropoda), and the like. 



In my Monograph en the Calcareous Sponges, which 

 appeared in 1872, 1 have shown that this same pair of primary 

 germ-layers forms the basis of the body of the Sponges, and 



