THE BREEDING HABITS OF ANIMALS 



195 



manders. Much more commonly, however, if the eggs undergo any 

 development at all within the mother, they remain until a rather late 

 larval stage, or until the form of the adult is attained. Some insects, 

 some snakes, and the true mammals are of the last-named type. Such 

 animals are not said to lay their eggs, since it is well advanced larvae 

 or embryos that are discharged. 



Source of Nourishment of the Embryo. — Animals that lay their eggs 

 are said to be oviparous; the eggs may be laid before fertilization, or, if 

 after fertilization, while the embryos are still incapable of existence out- 

 side of the egg membranes. Animals that retain the embryos until with 

 proper care they are capable of in- 

 dependent existence are designated 

 viviparous. Of viviparous species 

 there are two general types. In one 

 of these, the eggs are large and laden 

 with yolk, from which the embryo 

 derives its nourishment, just as in 

 oviparous animals. The mother 

 serves, in such cases, chiefly as a 

 nest in which the eggs may develop. 

 Viviparous animals in which practi- 

 cally the whole nourishment of the 

 young is furnished by the egg itself 

 are said to be ovoviviparous. Some 

 reptiles are ovoviviparous (Fig. 148), 

 the embryos being held in the oviduct Fi^. 148.— Urinogenital system of a 



' -11 r 1 lizard. 5, bladder; C'Z, cloaca; i?, kidney; 



of the mother until they are lar ad- q, ovary; Ov, oviduct; Oo\ cloacal open- 



vanced but receiving the food from i^s of oviduct; Ov\ abdominal opening 



, , . . of oviducts; R, rectum. The lizards are 



the egg. The second type Ot VlVip- oviparous or ovoviviparous. In ovo- 



aroUS animal is that in which the viviparous forms, those in which the 



, , • 1 i • 1 young reach an independent stage before 



nutrition ot the embryo is obtained birth and are not attached to the mother, 



from the mother, whose reproductive the development of the fertilized eggs 



. , , ' „ , , , , takes place in the oviducts. 



systerar is then oi the general type rep- 

 resented in Fig. 99. The blood vessels of the embryo, running through 

 the umbilical cord and expanding in a highly vascular tissue known as the 

 placenta, are brought into very close contact with the maternal blood 

 vessels in the wall of the uterus (Fig. 149). While no blood cells pass 

 from the vessels of the mother to those of the embryo, nutritive mate- 

 rials and oxygen in solution readily diffuse into the blood of the latter. 

 Formes in which the embrj^o is connected with the maternal uterus by 

 a placenta are spoken of as truly viviparous. Viviparity is found in 

 some insects, as in the plant lice, but it is not likely that the mother 

 furnishes any nutrition. The eggs are small, but there is nothing com- 

 parable to a placenta. Hydra and jellyfishes exhibit something like 



