During sexual reproduction, each parent animal must form specialized cells known as gametes, which are genetically re-combined and in which the chromosome number is reduced by half from a diploid double set to a haploid single set. Both processes occur during meiosis, which is the first stage of ga mete formation, or gametogenesis. In virtually all animals that reproduce sexually, the gametes occur in two morphologically distinct forms corresponding to male and female. These distinctions in form and structure are related to the specific functions of each gamete. The differences become apparent during the latter stages of spermatogenesis (for male gametes) and oogenesis (for female gametes).

After spermatogenic meiosis, the morphological transformation of the male gamete generally includes development of a small motile sperm. The sperm's function is to move toward and ultimately meet the female gamete, beginning a sequence of events that ends in the fusion of the two gametes. After the gametes fuse, the sperm's role is essentially complete except for the final genetic contribution from its half of the new offspring's genome. Thus, the primary structures developed by the sperm are concerned with movement and with engaging the female gamete and its coatings. The specific lo-comotory structures of sperm vary among the lower meta-zoans, ranging from pseudopodia (temporary extensions of cell material) resembling amoebas in the roundworms (phylum Nemata) to flagella (long whiplike projections) in most other groups.

After oogenetic meiosis, the morphological transformation of the female gamete generally includes development of a large oocyte that does not move around. The oocyte's functions are far more numerous than those of the sperm. For most lower metazoan groups, they include equal or greater participation in the process of gamete fusion. Following fusion, however, the oocyte must provide the coordination of and materials for all the early stages of embryo and larval de

A West Indies sea egg (Tripneustes ventricosus) releasing sperm into water. (Photo by ©Andrew J. Martinez/Photo Researchers, Inc. Reproduced by permission.)

velopment. To carry out these functions, the oocyte must build up large stores of energy-rich nutrients (e.g., carbohydrates in the form of glycogen, and lipid or proteinaceous yolk); phospholipid stores for membrane synthesis; extra nucleotides or redundant DNA; transcripts of RNA for protein synthesis; extra regulatory and structural proteins; and occasionally materials for eventual eggshell formation. In most cases, the oocyte must fulfill all these functions by itself. Some animals, however, use other germ cells to assist the oocyte. Perhaps the best example of this assistance is found among the parasitic flatworms (phylum Platyhelminthes). These organisms have special vitelline (resembling yolk) germ cells that never become gametes but instead supply the gametes with needed materials. The oogenic stage that is most often involved in fertilization is the oocyte; however, there are some exceptions to this generalization. The ambiguous term "egg" is often applied to oocytes and other fertilizable stages of female gametes. "Egg" may, however, also refer to fully formed embryos or juveniles within various embryonic coverings, so the word should be avoided in most instances.

Spermatogenesis and oogenesis most often occur in different individual animals known as males and females respectively. This differentiation of sexes is known as gonochorism. Alternatively, it is quite common for the same individual to produce both sperm and oocytes. This condition is known as hermaphroditism; it may involve either simultaneous or sequential production of sperm and oocytes. Oogenesis and spermatogenesis may occur in different gonads, namely ovaries and testes, or may occur in a single hermaphroditic gonad. Such lower metazoans as sponges (phylum Porifera) may lack distinct gonads, with gametes developing in normally somatic regions of the body. Whether through gonochorism or hermaphroditism, gametogenesis may occur throughout the adult life of the animal, as in parasitic flatworms (phylum Platy-helminthes). The more common pattern among the lower metazoans, however, is one of seasonal reproduction.

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