254 The Vertebrate Organ Systems 



demonstrated among the higher vertebrates. Both frogs and rabbits 

 have been produced without the necessity of a male parent. Another un- 

 usual situation occurs in a few forms where the larvae or immature 

 form is able to reproduce. The gallfly, Miastor, commonly reproduces 

 parthenogenetically when still a larva. This is known as paedogenesis. 



The number of eggs produced by the different species varies greatly. 

 In most forms, this number is directly correlated with the amount of 

 parental care given to the young. Many of the marine invertebrates and 

 vertebrates simply cast the eggs and sperm into the water. Except for 

 the fact that the males and females are in the same area, the fertilization 

 of the eggs is to a great extent dependent upon chance. Even after fer- 

 tilization, no care is given to the developing eggs or newly hatched 

 young; thus there is tremendous mortality. While a single animal of 

 this type may produce millions of eggs within a single year, the highly 

 developed mammal produces only a comparative few in a lifetime. In the 

 mammal, however, fertilization is assured and the developing young are 

 well protected. 



Those animals which lay eggs that subsequently hatch are known as 

 oviparous. In some forms such as the garter snakes, these eggs are 

 retained within the body of the female until hatched. These forms are 

 known as ovoviviparous. The extreme of parental care of the young is 

 that illustrated by the mammals where the fetus is actually nourished 

 through a complex connection with the mother. Such animals are vivi- 

 parous. 



Many complex behavior patterns often assure the fertilization of the 

 eggs and the subsequent care of the young. 



FUNCTIONS OF THE REPRODUCTIVE SYSTEM 



Two interrelated functions may be considered as being characteristic 

 of this system. 



1. The Production of New Individuals. The primary function 

 of the reproductive system is to assure the continuation of the species by 

 providing the means for the production of new individuals. 



2. Genetic Variability. In all forms where cross fertilization 

 occurs, great variability may exist within the progeny. This allows for 

 the production of more vigorous individuals and also allows for chance 

 variations that may prove favorable. 



