128 How Living Things Come to be What They Are 



absorbed. The gills are also absorbed. An air sac connected 

 with the food pipe is present in both fish and tadpole. It 

 becomes in one case a pair of lungs but remains in the fish 

 merely an air bladder. 



The frog, on its way from being a one-celled water ani- 

 mal (the e^^) to being a land animal with four legs and a 

 pair of lungs, resembles a fish on its way from being a one- 

 celled animal (the fish Qgg) to being a backboned water- 

 inhabiting gill breather. This resemblance persists for only 

 a part of the individual's development. However, so far as 

 this resemblance does persist we infer a common ancestry. 



Gill-bearing Land Animals 



In addition to the fish and amphibians there are three 

 great classes of backboned animals — the reptiles, birds and 

 mammals. Any individual member of one of these classes, 

 in common with the fish and the frog and in common with 

 all other animals, starts upon life as a single cell, the fer- 

 tilized Q^^. The reptiles usually deposit their eggs, with 

 more or less of a horny shell containing more or less concen- 

 trated food, on the ground or in a burrow; and leave the 

 hatching to the heat of the sun and good fortune. The birds 

 typically give birth to their offspring as minute eggs enclosed 

 in limy shells containing a comparatively large mass of re- 

 serve food (the yolk and the albumen) and typically supply 

 protection and heat during the development of the individual 

 within the shell. In the mammals the minute q^^ develops 

 within the body of the mother until the embryo is at a 

 relatively advanced stage. 



In all three classes the eg^ divides into two cells, each of 

 which again divides into two making four, and each of these 

 divides, and so on, until there is formed a thin layer of cells 

 which gradually becomes elongated so that it is possible to 

 distinguished the head end from the hind end. This layer of 

 cells doubles and wrinkles and folds, acquiring a distinct ap- 

 pearance, but not resembling any known animal. If we com- 



