AN INTRODUCTION TO ANIMAL BIOLOGY 63 



blood-cells continue this process of disintegration and re- 

 moval of tissues until the tail has entirely disappeared. 

 Thus instead of the tail dropping off, as is commonly believed, 

 its substance is gradually carried back into the body by 

 internal processes of absorption, and the materials are 

 used in building other tissues. 



62. Embryonic and Larval Development. The develop- 

 ment of any fertilized egg to the hatching stage is called em- 

 bryonic development ; and the scientific study of such stages 

 of the frog and other animals is animal embryology. The 

 tadpole is often called a larva, and its changes constitute 

 larval development. 



All species of backboned animals have embryonic develop- 

 ment ; but a large number do not have a larva, for their eggs 

 develop directly into minature animals resembling the adults. 

 For example, it is well known that birds are hatched as 

 small birds, and common mammals at birth show the charac- 

 teristics of their species. All young animals which at hatch- 

 ing are quite unlike their parents (e.g., as tadpoles are unlike 

 frogs, and caterpillars are unlike butterflies) are said to have 

 a larval development. 



63. Oviparous and Viviparous Development. All such 

 animals as frogs, birds, turtles, fishes, etc., which lay eggs 

 that develop outside the body of the animal that produced 

 them, are called oviparous. Such eggs are usually protected 

 by special coverings or shells. We shall see later that 

 such oviparous development is very common in the animal 

 kingdom. 



In many species of animals eggs are retained in the oviducts 

 or other specialized cavities until embryonic development 

 is completed, and the young animals are " born alive " ; by 

 which we mean that as organisms ready for an independent 

 existence they are expelled from the cavity in which embry- 

 onic development occurred. Such internal development is 

 viviparous. It occurs in all mammals, except the duck- 



