682 LIFE : OUTLINES OF GENERAL BIOLOGY 



of the faint light of the great depths. Now the remarkable fact is 

 that the "telescope eyes" of various types of deep-water fishes are 

 closely paralleled by those of certain deep-water cuttlefishes, which 

 belong to the phylum of molluscs. On two entirely different lines of 

 evolution the same adaptation has been evolved; and the case is 

 the more striking since the eyes of fish and cuttlefish develop in 

 quite different ways, and are separated by important differences in 

 detailed structure. The convergence that we are emphasising here 

 Ls in the "telescope" structure; but there is also a remarkable 

 superficial resemblance between fish eyes and cuttlefish eyes in 

 general, though they develop in very different ways. For the fish 

 eye is a "brain-eye", growing outwards from the brain to the skin, 

 whereas the cuttlefish eye is a "skin-eye", beginning as a super- 

 ficial insinking of skin. In the chapter on Evolution there is some 

 consideration of the problem raised by the fact that the same 

 general result may be reached in unrelated types by different modes 

 of development. 



Some other instances of convergence may be simply mentioned; 

 the volplanes of various unrelated swooping mammals, among 

 marsupials, rodents, and insectivores ; the electric organs of the 

 Torpedo and the Electric Eel, the one a cartilaginous fish or 

 Selachian and the other a bony Teleost ; the parachutes formed by 

 the enlargement of the pectoral fins in unrelated "flying fishes", 

 Exocoetus and Dactylopterus ; the superficiad resemblance between 

 small rodents like mice and small insectivores like shrews, both 

 adapted to making their way through narrow holes and timnels. 



DEVELOPMENT OF ORGANS.— In studying homologies, which 

 form the basis for sound classification, it is necessary to consider 

 the mode of development, and it may be useful here to introduce 

 the general grouping for Vertebrate animals, (i) The outer germinal 

 layer (the ectoderm or epiblast) gives rise to the epidermis and the 

 structures it bears, to the nervous system, and to the foundations 

 of the sense-organs. (2) The inner germinal layer (the endoderm or 

 hypoblast) gives rise to the lining of the food-canal and of all its 

 outgrowths (lungs, liver, pancreas, etc.), and to the skeletal rod, 

 the notochord, which is folded off along the dorsal median line of 

 the embryonic gut or archenteron. (3) The middle germinal layer 

 (the mesoderm or mesoblast) forms skeleton, muscles, connective 

 tissue swathings, the lining of the body-cavity or coelom, the 

 muscular and connective sheath surrounding the food-canal, and 

 such important organs as heart and kidneys. When an endodermic 

 pouch grows out from the embryonic food-canal it will carry the 

 ensheathing mesoderm with it, and thus all organs, like lungs and 

 liver, that arise as diverticula of the mid-gut or digestive canal will 

 be compounded of endoderm more internally and mesoderm more 



