XXI.] EMBRYOLOGY. 255 



the rest have originated ; but where the higher forms of 

 animals are developed by a similarly indirect process, 

 their development, when the germinal mass first begins 

 to differentiate into tissues, begins not on the plan of a 

 Protozoon, but on that of a lower form of the same 

 fundamental type to which the perfect form belongs. 



This remarkable fact of a change in the plan of develop- 

 ment is well known in particular cases, under the name 

 of metamorphosis ; the most familiar instance, and one of Insect 

 the best, is that of the transformation of the worm into the phosi"°'" 

 fly, or of the caterpillar into the butterfly. But meta- 

 morphosis is not confined to such obvious cases as these ; 

 it is universal among the air-breathing Vertebrata : in most 

 of which, however, the metamorphoses are finished before Metamor- 

 the animal leaves the egg or the womb. All winged f-^^f^-^^ 

 insects undergo metamorphosis, though some u.ndergo birth. 

 much greater metamorphoses than others. Some leave 

 the egg in the six-legged form of a mature insect, only 

 without wings ; others in a many-legged form like 

 centipedes : these are the caterpillars. Others, again, leave 

 the egg in the form of worms, without any legs, in which 

 state they are called maggots. 'Now, insects are the Insect 

 highest class of the Articulata, centipedes are a lower ^.esembie 

 class of that great group, and worms are a still lower J,°^^''i' 

 class of the same than centipedes ; so that the larval Articulata. 

 forms of insects resemble the mature forms of those lower 

 classes to which they are allied. The same is true of So of 

 the metamorphoses of the Batrachians (frogs, newts, &c.). 

 These are air-breathing Vertebrates, and air-breathing 

 organisms are, in general, higher than the water-breathing 

 forms to which they are most nearly allied ; but the 

 larvse, or tadpoles, of the Batrachians are water-breathers, 

 and have branchiae which are altogether homologous with 

 those of fishes. The resemblance of tadpoles to fishes, Larvse 

 however, is not to mature fishes, but to young or embryonic [^mature 

 ones ; and the same is true of insect larvae : they do not low forms, 

 resemble mature worms or centipedes, so much as imma- 

 ture ones.^ To express this law in the most general terms : 

 1 Carpenter's Comparative Physiologj^ p. 581. 



