Fig. 21-2. Some vertebrates, such as the fish, the frog, and the bird, lay eggs (oviparous) which 

 hatch outside the body. Others, such as the shark, retain the eggs within the uterus (ovovivip- 

 arous) where they hatch, and the young are born in an active condition. Still others, such as 

 the mammals, produce young from small eggs (viviparous) and the young receive nutrients 

 from the uterine wall. 



