74 AN INTRODUCTION TO ZOOLOGY. 



way of providing for the uncertainty which exists as 

 to whether, in these cases, it is altogether conaparable 

 with the mesoblast of other types. Animals develop 

 to a higher point, in which the body layers develop 

 complicated organs, usually go through a larval stage 

 very different in. appearance from the adult. 



When an animal leaves the e^^ in a form fairly 

 similar to that of the adult, it is said to develop 

 directli/ ; when it is of an entirely different shape and 

 appearance, and has to lose some of its parts, or gain 

 others, before it grows up, it is said to undergo meta- 

 morphosis or change of form, and the early stage 

 is spoken of as the larva. The development of the 

 butterfly is the most familiar instance of this ; it 

 changes twice, from the worm stage or caterpillar to a 

 resting stage, called the pupa, whence issues the 

 im.ago, or perfect insect. Most larvse, however, do 

 not present a pupa stage. The vast majority of the 

 invertebrata undergo metamorphosis; it is only among 

 the vertebrata that we find direct development the 

 rule; but even there the Amphibia, which have a 

 larval stage, are an exception. Animals with direct 

 development may, however, be more correctly de- 

 scribed as undergoing metamorphosis within the egg, 

 than as having no metamorphosis; for great and start- 

 ling changes in their structure take place within the 

 egg, although these are not so strikingly apparent to 

 the observer as when there is a free larval stage. The 

 young animal enclosed within an egg (or, in mammals, 

 before birth) is called the Em.bryo. 



