. Principles of modern biology. Biology. 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
. Principles of modern biology. Biology. 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
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