464 STRUCTURE OF VERTEBRATA. 



fitted, tends to prevent a speedy establishment of the adult form. 

 When much yolk is present it usually forms a hernia-like yolk sac, hang- 

 ing down from the embryonic gut. As a further consequence, we may 

 notice the tendency to the. production of embryonic organs useful only 

 during embryonic life. We must consider the formation of an organic 

 connection between mother and unborn young as a further step in the 

 same direction as the acquisition of yolk. This is hinted at in some 

 Fishes and Reptiles, but culminates in the placental Mammals. It may 

 be looked at in two different ways. On the one hand, the diversion 

 of the nourishment from the ovary, during the period of gestation, 

 tends to starve the remaining ovarian ova, and this check to fertility 

 is further prolonged during lactation (Ryder) ; on the other hand, the 

 chance of survival is much increased, and the maternal sacrifice finds 

 its justification in the increased specialisation of the offspring. 



In accordance with the effect of the presence of yolk as noted above, 

 we find that segmentation is total (holoblastic) in the ova of the lam- 

 prey, the sturgeon, Ceratodus, Amphibians, and all Mammals except the 

 Monotremes. In the ova of Elasmobranchs, Teleosteans, Reptiles, 

 Birds, and Monotremes, the activity of the protoplasm is not sufficient 

 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 (recently 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, 

 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. 



