DEVELOPMENT OF VERTEBRATES. 401 



A gastrula is formed, in part at least, by distinct invagina- 

 tion in the development of the lamprey, the sturgeon, and 

 Amphibians ; it is more modified in Teleosteans and Elas- 

 mobranchs whose ova have more yolk ; it is much disguised 

 in Sauropsida and Mammals. 



Most Vertebrates lay eggs in which the young are hatched, 

 and these animals are usually called oviparous, though 

 all animals do of course produce eggs. In some sharks, 

 a few Teleosteans, some tailed Amphibians, a few lizards 

 and snakes, the young are hatched before they leave the 

 body of the mother animal. To such the awkward term 

 ovo-viviparous is sometimes applied, but there is no real 

 distinction between this mode of birth and that called 

 oviparous, and both may occur in one animal (e.g., in the 

 grass snake) in different conditions. In the placental Mam- 

 mals, there is a close organic connection between the unborn 

 young and the mother, and the parturition in this case is 

 usually called viviparous. But this term is also objection- 

 able, since all animals are in a sense viviparous. 



2 c 



