142 Darwin, and after Darwin. 
have just seen, remain permanently as large gastrule, 
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, cach taking 
a different road in their subscquent evolution. I will 
therefore confine attention to only one of these 
several roads or methods, namely, that which is 
