142 Darwin, and after Darwin. 



have just seen, remain permanently as large gastrulae, 

 with sundry specialized additions in the way of 

 tentacles, &c. This stage of differentiation consists in 

 the formation of either a pouch or an additional 

 layer between the ectoderm and the endoderm, which 

 is called the mesoderm. It is probably in most cases 

 derived from the endoderm, but the exact mode of its 

 derivation is still somewhat obscure. Sometimes it 

 has the appearance of itself constituting two layers ; 

 but it is needless to go into these details ; for in any 

 case the ultimate result is the same — viz. that of con- 

 verting the Metazoon into the form of a tube, the walls 

 of which are composed of concentric layers of cells. 

 The outermost layer afterwards gives rise to the 

 epidermis with its various appendages, and also to the 

 central nervous system with its organs of special sense. 

 The median layer gives rise to the voluntary muscles, 

 bones, cartilages, &c., the nutritive systems of the 

 blood, the chyle, the lymph, and the muscular tube 

 of the intestine. Lastly, the innermost layer deve- 

 lopes into the epithelium lining of the intestine, 

 with its various appendages of liver, lungs, intestinal 

 glands, &c. 



I have just said that this three or four layered stage 

 is shared by all the Metazoa, except those very lowest 

 forms — such as sponges and jelly-fish — which do not 

 pass on to it. But from this point the developmental 

 histories of all the main branches of the Metazoa 

 diverge — the Vermes, the Echinodermata, the Mol- 

 lusca, the Articulata, and the Vertebrata, each taking 

 a different road in their subsequent evolution. I will 

 therefore confine attention to only one of these 

 several roads or methods, namely, that which is 



