306 REPRODUCTION IN ANIMALS 



gress from a condition where all appear to be essentially 

 alike in form and structure. They usually move about 

 to a greater or less degree in assuming their proper posi- 

 tions, and it is only toward the close of development that 

 they assume the characteristic shape and organization of 

 the various tissues. Development and differentiation 

 thus proceed at an equal pace. 



Metamorphosis. — The process of development has 

 been pictured as advancing by gradual stages from the 

 egg to the adult. There are many animals, however, in 

 which such is not the case. The young butterfly, for ex- 

 ample, hatches from the egg as a crawling worm-like 

 creature, the caterpillar or larva, that, after feeding on 

 leaves and increasing in size, encloses itself in a web or at 

 least becomes a quiet pupa; when certain complicated 

 changes have ensued within the body, the pupal case 

 bursts and a wholly different type of individual emerges 

 fitted for a life on the wung and a diet of nectar. This 

 marked change of form and function is known as a meta- 

 morphosis (Fig. 74). Fly larvae or maggots, the 

 '* wrigglers " of mosquitoes, the young of starfishes, sea 

 urchins, crabs and worms are frequently so unlike the 

 adults that it is only after the metamorphosis that their 

 relationship can be determined. 



Among the vertebrates, the frogs and toads undergo a 

 complicated metamorphosis whereby the fish-like water- 

 inhabiting tadpole becomes transformed into the air- 

 breathing, four-legged and tailless land animal. In many 

 eels the change from the transparent flattened larva to 

 the round pigmented adult is likewise very marked. 

 Finally, there are many larvtc, especially in the sea, whose 

 life histories are unknown and whose structure is so un- 

 like that of any known adult that their position in the 

 animal kingdom is problematical. 



Care of Eggs. — Among many of the lower animals, 

 such as starfishes, sea urchins, and many worms, the eggs 

 are laid in the water and are forthwith abandoned by the 



