THE REPRODUCTION OF ANIMALS 



229 



outwardly the appearance of a viviparous one. This type of reproduc- 

 tion is termed ovoviviparous. In some of the sharks another type of ovovivi- 

 parous habit is shown that is more nearly like true viviparity. In this 

 case, the egg has a fairly large yolk but no white or shell, and, when the 

 yolk has all been utilized, the embryo develops a belated placental con- 

 nection with the mother's tissues. 



Even when embryonic development is complete and the young are 

 born or hatched, they still have to undergo a more or less prolonged 



Fig. 15.10. Parental care. Female, nest, and young of the ruby-throated hummingbird. 

 (Photo by Allan D. Cr uickshank, courtesy National Audubon Society.) 



period of postembryonic development before they reach the adult con- 

 dition. In nearly all animals this is a period of high mortality, and a 

 large proportion, often a great majority, of the young perish. In general, 

 animals have adopted one or both of two methods of offsetting this 

 loss — a very high reproductive rate or some type of parental care of the 

 young, at least while they are in the most helpless part of this period. 



In the higher vertebrates and in most of the fishes, where the egg 

 is either well supplied with food (strongly telolecithal) or viviparity 

 occurs, the young are hatched or born in a highly developed condition 

 and with a clear resemblance to the adult. Even among these forms, 

 however, there is great variation as to the degree of precociousness or 



