THE BREEDING BEHAVIOR OF ANIMALS 



183 



-Ovi 



Ov_ 



well-formed embryo is produced, and then the eggs are laid; this occurs 

 in some of the salamanders. Usually, if the eggs undergo more than a 

 few cleavages while within the mother, they remain until a rather late 

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

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



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 the egg membranes. Animals that retain the embryos until with 

 proper care they are capable of independent 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 prac- 

 tically the whole nourishment of the 

 young lis furnished by the egg itself 

 are said to be ovoviviparous. Some 

 reptiles are ovoviviparous (Fig. 152), 

 the embryos being held in the oviduct 

 of the mother until they are far 

 advanced but receiving the food from 

 the egg. The second type of vivipa- 

 rous animal is that in which the 

 nutrition of the embryo is obtained 

 from the mother, whose reproductive 

 system is then of the general type 

 represented in Fig. 148. The embryo, 

 resting in the uterus, has as intimate a relation with the mother's blood 

 vessels as do the mother's own tissues. Blood vessels of the embryo 

 extend out through the umbilical cord, and branch profusely at the end 

 in a highly vascular structure known as the placenta (Fig. 153, left). 

 The placenta is furnished partly by the embryo, partly by the uterus of 

 the mother. In it the blood of the mother and that of the embryo, while 

 never joining in the same vessels, are separated only by the thin walls of 

 their respective capillaries. In the human placenta the connection is 

 even closer, for the walls of the maternal vessels become eroded away, so 

 that the blood comes to lie in large sinuses, resembling the open blood 

 spaces of the crayfish or insect circulatory system (page 122). In this 

 lake of maternal blood the capillaries of the fetal system (branches of the 



Fig. 152. — Urinogenital system of a 

 lizard. B, bladder; CI, cloaca; K, kid- 

 ney; 0, ovary; Ov, oviduct; Ow^, cloaca! 

 opening of oviduct; Ov^, abdominal open- 

 ing of oviducts; R, rectuni. The lizards 

 are oviparous or ovoviviparous. 



