REPRODUCTION 125 



during her lifetime, in her "nuptial flight." The sperm are stored in a 

 pouch connected with the genital tract and closed by a muscular valve. 

 If sperm are released from the pouch as she lays eggs, fertilization occurs 

 and the eggs develop into females— queens and workers. If the eggs are 

 not fertihzed they develop into males— drones. 



Changes in temperature, in pH or in the salt content of the sur- 

 rounding water, or chemical or mechanical stimulation of the egg itself 

 will stimulate many eggs to parthenogenetic development. A variety 

 of marine invertebrates, frogs, salamanders, and even rabbits have been 

 produced parthenogenetically. The resulting adult animals are gen- 

 erally weaker and smaller than normal, and are infertile. 



The females of all birds, most insects, and many aquatic inverte- 

 brates lay eggs from which the young eventually hatch; such animals are 

 said to be oviparous (egg-bearing). In contrast, mammals produce small 

 eggs which are kept in the uterus and provided with nutrients from the 

 mother's blood until development has pioceeded to the stage where they 

 can exist independently, to some extent at least. Such animals are said 

 to be viviparous (live-bearing). In certain other forms— some insects, 

 sharks, lizards and certain snakes— the female is ovoviviparous, she pro- 

 duces large, yolk-filled eggs which are retained within the female 

 reproductive tract for a considerable period of development. The de- 

 veloping embryo forms no close connection with the wall of the oviduct 

 or uterus and receives no nourishment from the mother. 



The number of eggs produced by each female of a given species 

 and the chance that any particular egg will survive to maturity are in- 

 versely related. In the evolution of the vertebrates from fish to mam- 

 mals, the trend has been towards the production of fewer eggs, and the 

 development of instincts for better parental care of the young. Fish 

 such as the cod or salmon produce millions of eggs each year, but only 

 a small number of these ever become adult fish; in contrast, mammals 

 have few offspring but take good care of them so that the majority 

 attain maturity. Fish and amphibia generally take no care of develop- 

 ing eggs, which are simply deposited in water and left to complete de- 

 velopment unaided. The eggs of reptiles are usually laid in earth or 

 sand and develop there without parental care, warmed by the sun. 

 Birds, in contrast, have a complex behavior pattern for nest-building, 

 incubating the eggs by sitting on them and caring for the newly 

 hatched youngsters. The mammalian egg develops within the mother's 

 uterus where it is safe from predatois and from harmful factors in the 

 environment. Most mammals have a strong "maternal instinct" to take 

 care of the newborn until they can shift for themselves. 



Many animals have other special types of instinctive behavior, or 

 "breeding habits" to insure successful reproduction. A number of 

 vertebrate and invertebrate species have characteristic courting and 

 mating behavior patterns which may be dangerous or even fatal to the 

 individual, yet insure the continuation of the species. Salmon swim 

 hundreds of miles upstream to spawn and die, male spiders are fre- 

 quently eaten by the females after fertilizing them, and so on. 



