MULTIPLICATION AND DEVELOPMENT 83 



Birth and hatching. — When a young animal is born 

 alive, it usually resembles in appearance and structure the 

 parent, although of course it is much smaller, and requires 

 always a certain time to complete its development and be- 

 come mature. A young kangaroo or opossum is carried 

 for some time after its birth in an external pouch on the 

 mother's body and is a very helpless animal. A young 

 kitten is bom with eyes not yet opened and must be fed by 

 the mother for several weeks. On the other hand young 

 Rocky Mountain sheep are able to run about swiftly within 

 a few hours after birth. 



Most animals appear first as eggs laid by the mother. 

 This is true of the birds, the Teptiles, 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 embryonic 

 development; and the development from then on, for ex- 

 ample, that of the chick to adult hen or rooster, or that of 



