placenta which is attached by folds to the uterine wall and 

 which absorbs materials from the maternal blood (Figure 

 7-16). Many teleosts are viviparous (live-bearing) and 

 show the same range of adaptations as the sharks. 



.Some other terms can be introduced here which apply to 

 the conditions described. Oviparous describes those animals 

 which lay eggs. Ovoviviparous describes the retention of 

 eggs within the reproductive tract, or some specialized area 

 (pits in skin, vocal sacs, or brood pouches), until the young 

 hatch or complete their yolk-sustained development. Ovo- 

 viviparity grades into "viviparity." In the latter, the egg is 

 usually yolk-poor, the developing embryo absorbs nutritive 

 materials secreted by the uterine wall or has a very close 

 tissue relationship with the uterine wall so that materials 

 can be absorbed directly from the blood. This type might 

 better be described as metaviviparous since viviparous only 

 infers being born alive. From these definitions it follows that 

 some sharks and teleosts are ovoviviparous, while others are 

 metaviviparous. 



In amphibians and reptiles viviparity sometimes occurs 

 and a variety of relationships exists between the young and 

 the parent. Among amphibians there is no indication of the 

 absorption of nutritive materials. The opposite extreme is 

 seen in some reptiles. In the lizards Chakhides and Ti/iqua, a 

 kind of placentation similar to that of the mammal is 

 observed (Figure 7-17). 



mesentery 



uterus 



.chorion frondoso-plocenta 

 allcntois 

 amniotic cavity 



uterine cavity 



-extraembryonic coelom 

 yolk sac 



Figure 7-17. Chorioollontoic placenta of the lizard, Tiligua scinco/des. 

 (After deLonge) 



Mammals range from the nonplacental monotremes (ovip- 

 arous to the highly developed placentals. In the marsu- 

 pials the transition from ovoviviparous to metaviviparous is 

 seen. In the former, the yolk sac is large and its blood sup- 

 ply modified. The embryo sinks down and is nearly enclosed 

 in the yolk mass; the allantois develops as a finger-like pro- 

 jection down into the yolk mass. The vitelline veins on the 

 surface of the yolk pass to either side of the neck of 

 the allantoic sac (Figure 7-18). 



uterine wall 



maternal blood vessels 



sacciform gland mucous-filled glands 



Figure 7-16. Yolk-soc placenta of Mustelus canis. (Mainly after Ranzi) 



PLACENTA 



211 



