TESTIS 



OVARY 



Fig. 21-1. Sexually mature Hydra, showing the tem- 

 porary gonads. 



Finally only one hugely swollen cell remains, 

 and this is the egg. Fertilization occurs after 

 the epithelial wall of the ovary has disinte- 

 grated and one of the free-swimming sperm 

 chances to make contact with an exposed 

 egg. Cross-fertilization is assured by the fact 

 that the eggs and sperm of any one indi- 

 vidual do not reach maturity simultaneously. 

 Temporarily, while the egg is cleaving to 

 form the blastula, the embryo remains at- 

 tached to the body of the parent; but at 

 about the time of gastrulation, the offspring 

 develops cilia and swims away as an inde- 

 pendent larva. 



The monoecious (hermaphroditic) condi- 

 tion, in which both testes and ovaries are pos- 

 sessed by one individual, is not uncommon, 

 especially among invertebrate animals; but 

 on the whole, dioecious species, in which the 



Reproduction in Multicellular Animals - 381 



sexes are clearly separate, greatly predomi- 

 nate. Also a vast majority of animals possess 

 truly permanent gonads of greater or lesser 

 complexity. 



In addition to the gonads most animals 

 possess various accessory reproductive struc- 

 tures. Usually there are two or more sperm 

 ducts, through which the sperm are con- 

 ducted from the testes, and oviducts, which 

 convey the eggs from the ovaries. Also one 

 may find a variety of glands and other spe- 

 cial structures, closely associated with either 

 or both of the gonoducts. 



EXTERNAL VS. INTERNAL FERTILIZATION 



As a general rule aquatic animals merely 

 liberate the gametes into the surrounding 

 water and the coming together of the sperm 

 and eggs is more or less a matter of chance. 

 However, in many cases the males are at- 

 tracted to the vicinity of where the females 

 are discharging the eggs and are excited to 

 discharge the sperm simultaneously. Usually 

 such externally fertilized eggs receive little 

 or no parental care during development, but 

 in some cases (Fig. 21-2) either or both par- 

 ents may stand guard over the eggs. In some 

 cases also the fertilized eggs become ce- 

 mented to the body of one of the parents, 

 which carries and protects the embryos dur- 

 ing development. 



Some essentially nonaquatic animals— such 

 as many amphibians and insects— return to 

 the water to lay their eggs, which may be 

 fertilized either before or after discharge. 

 Most terrestrial forms, however, ejaculate the 

 sperm directly into the reproductive tract of 

 the female, and thus fertilization occurs in- 

 ternally. Typically this process involves copu- 

 lation between the sexes, and frequently the 

 male is equipped with a penis, or some other 

 type of intromittent organ. The ejaculate, 

 or semen, includes not only the sperm, but 

 also a greater or lesser amount of aqueous 

 fluid secreted by the sperm ducts and asso- 

 ciated glands. This fluid provides a suitable 

 medium for the sperm to swim in, and also 



