The gonads, ovaries and testes, of all vertebrates are similar in structure and function. The gonads are usually paired (although they may be fused or single in some groups). The testes in all vertebrates produce millions of sperm at a time. As we shall see, the major differences in male reproductive systems involve the mechanisms for transferring those millions of sperm to the ova.
Ovaries produce ova surrounded by fluid filled sacs called follicles. These follicles burst releasing ova into the coelom and then the oviduct. The number of ova produced at any one time varies considerably depending upon the habitat of the animal and the amount of parental care the young will receive. Thus fish, like the cod, which release unprotected eggs into the ocean and do not care for their young may produce and release more than a million eggs at once. Other fish and amphibians which do not broadcast their ova as far afield or which give some measure of parental protection to the eggs may produce thousands of eggs at once. Birds and mammals which invest considerable parental energy in protecting their offspring both before and after birth produce only a few eggs (ova) at one time.
Once ova are released from the ovary, they are transported through the oviduct toward the exterior. In the case of fish and amphibians, which release shell-less eggs, the oviduct is essentially an unspecialized tube leading indirectly to the exterior. Birds and reptiles produce shelled eggs; the shell is critical in protecting the egg from desiccation in the terrestrial environment. The oviducts in these animals are modified, being in close association with a shell-forming gland and capable of considerable distension to accommodate the enormous, inflexible eggs that result.
Mammals which bear their young live, have developed a different modification of the oviduct system. The distal portion of the duct, involved in shell deposition in birds and reptiles, is enlarged as the uterus. This thick walled muscular structure, the lining of which will form an intimate relationship with the membranes around the embryo to form the placenta, will participate in nourishing the young as they develop within the female reproductive tract. In primitive mammals (the rat is a fine example), the two uterine horns remain separate; in higher mammals (primates) which produce fewer, more developed young, the two horns of the uterus are fused to form a single large uterus.
A critical difference between vertebrates which release unprotected eggs and those which release shelled eggs or in which the young develop within the female's body, is the timing and location of fertilization. In fish and amphibians which release unshelled eggs into an aquatic environment, fertilization is external, occurring in the water in which the males and females release their gametes. Embryonic development, as well as fertilization, will take place in the aquatic environment. No special organs are required for transfer of sperm although in some cases the males may have structures such as thumb pads of frogs for holding the female in place so sperm and eggs are more likely to be in the same place at the same time.
In animals with shelled eggs or internal development, fertilization must be internal and these animals need a mechanism for getting the sperm to the egg while the egg is still in the appropriate region in the female's body. Interestingly, birds lack copulatory organs and rely on close apposition of males and female genital openings for sperm transfer. Male reptiles and mammals, have accessory structures (the penis) which can be inserted into the female genital opening releasing the sperm well within the female's body.
In many ways the problems of reproduction in aquatic and terrestrial environments are similar for plants and animals. As a result, many of the solutions are similar.