62 ELEMENTARY ZOOLOGY 



Most animals appear first as eggs laid by the mother. 

 This is true of the birds, the reptiles, the fishes, the 

 insects, and most of the hosts of invertebrate animals, 

 This egg may be cared for by the parent as with the 

 birds, or simply deposited in a safe place as with most 

 insects, or perhaps dropped without care into the water as 

 with most marine invertebrates. The young animal which 

 issues from the egg may at the time of its hatching 

 resemble the parent in appearance and structural character 

 (although always much smaller) as with the birds, some 

 of the insects, and many of the other animals. Or it may 

 issue in a so-called larval condition, in which it resembles 

 the parent but slightly or not at all, as is the case with 

 the gill-bearing, legless, tailed tadpole of the frog or the 

 crawling, wingless, wormlike caterpillar of the butterfly, 

 or the maggot of the house-fly. 



Life-history. Any animal which hatches from an egg 

 has undergone a longer or shorter period of development 

 within the egg-shell before hatching. The development 

 of an animal from first germ-cell to the time it leaves the 

 egg, for example, the development of the embryo chick 

 from the first cell to time of hatching, is called its em- 

 bryonic development; and the development from then on, 

 for example, that of the chick to adult hen or rooster, 

 or that of tadpole to frog, is called the post-embryonic 

 development. Beginning students of animals cannot 

 study the embryonic development (embryology} of animals 

 readily, but they can in many cases easily follow the 

 course of the post-embryonic development, and this stud}' 

 will always be interesting and valuable, When the 

 " life-history " of an animal is spoken of in this book, or 

 other elementary text-book of zoology, it is the history 

 of the life of the animal from the time of its birth or 

 hatching to and through adult condition that is meant, 

 not the complete life-history from beginning single egg- 



