REPRODUCTIVE SYSTEM. 495 



to overcome the inertia of the yolk, and segmentation is partial 

 (meroblastic). 



Similarly we find that a gastrula is formed, in part at least, by distinct 

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

 Amphibians (though the occurrence of invagination has been denied for 

 the frog) ; it is more modified in Teleosteans and Elasmobranchs, whose 

 ova have more yolk ; it is much disguised in Sauropsida and Mammals. 



Most Vertebrates lay eggs in which the young are hatched 

 outside of the body, and to all these forms the term ovi- 

 parous is applied. 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. To 

 these cases the awkward term ovo-viviparous is 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 Mammals there is a close organic connection 

 between the unborn young and the mother, and the 

 parturition in this case is usually called viviparous. But all 

 the three terms are bad. 



