are brought together and to the protection and nourishment of the new 

 individual. 



Aquatic Vertebrates^ Among the fishes the female usually deposits the 

 eggs in quiet places at the bottom of the sea, near shore, or in quiet pools 

 of rivers. Then the male swims over the eggs, dropping a quantity of the 

 seminal fluid which contains sperms (see illustration, p. 380). The countless 

 sperm cells swarm about the heavier egg cells. One of the many sperm cells 

 swimming around a particular egg forces itself through the covering membrane, 

 and the fusion of the two cells takes place. As soon as the nucleus of the male 

 gamete and the nucleus of the female gamete have united, the combined 

 nucleus begins to divide, and so the development of a new fish is started. 



Whether or not the sperm cells are attracted to the eggs by specific chemi- 

 cal tropisms, two conditions favor fertilization: (1) both gametes are dis- 

 charged into the water in the same region and at about the same time; (2) the 

 proportion of sperm cells to egg cells is enormous. 



The ^gg cell of the fish contains a quantity of food material in addition to 

 the living protoplasm. The young fish developing out of the fertilized egg 

 lives on this accumulated food. In some species of fish one or both parents 

 swim about in the neighborhood of the developing fry and protect them against 

 destruction by their natural enemies. 



Reproduction among Amphibians- In the common frog the male and 

 the female are not ordinarily distinguishable. During the breeding season, 

 however, the ovary becomes very much enlarged as the eggs are being formed, 

 so that the female is rather swollen. In the spring the adult frogs come out 

 of their winter sleep and move to the ponds. Near the edge of the pond a 

 male gets on the back of a female and clasps her firmly with the front legs. 

 During this copulation, or joining, the eggs emerge from the female, enclosed 

 in a mass of gelatinous slime; at the same time the male discharges the seminal 

 fluid over the emerging eggs. Fertilization thus takes place in the water. 

 The parent frogs swim off and pay no further attention to the fertilized tggs 

 or to one another. In some species of amphibians, however, there is a great 

 deal of parental care (see page 421). 



Among all the vertebrate animals above the amphibians the &ggs are ferti- 

 lized while they are still inside the mother's body. But internal fertilization 

 takes place also among several groups of amphibians and even among fish. 

 The little guppy, a tropical fish often cultivated in home aquariums, is an ex- 

 ample of a viviparous species; that is, one in which the female brings forth 

 "living" young, in distinction from oviparous species, which are "egg-bearing". 

 Now most water animals discharge their eggs into the water, where they are 

 fertilized by the swimming sperms. The oviparous reptiles and birds, as well 



iSee No. 7, p. 395. 

 2See No. 8, p. 396. 



378 



