Vertebrate Reproduction

II. General features of vertebrate reproduction.

C. Polygamy versus monogamy.

D. Oviparity, ovoviviparity, and viviparity.

The preservation of species requires reproduction. Among the vertebrates, a diverse array of reproductive mechanisms have evolved. These include both sexual and asexual fertilization events. Moreover, offspring may be produced by oviparous, ovoviviparous, and viviparous processes. The different vertebrate classes additionally exhibit considerable variation in the amount of care which is provided to the young. Relatively complex mechanisms for nurturing offspring occur in the Aves and Mammalia classes.

Most vertebrate species have separate sexes. Genetic variation is the "raw material" of evolution (6:593). Sexual reproduction ensures the combination of different genotypes. In male vertebrates, the testes produce spermatozoa; in females, the ovaries produce eggs. For the most

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ostomata, does have a jaw mouth. The biting jaw developed about 395 million years ago, towards the end of the Silurian period. It enabled vertebrates to become more active carnivores. Two important Gnathostomata groups which arose in the late Devonian include the cartilaginous fishes (Chondrichthyes) and the bony fishes (Osteichthyes) (12:114). Elasmobranchii, or "plate-gilled" fishes, are a widespread group (12:137). They are nearly all carnivores or scavengers, and include the sharks, skates, and rays. The genital systems of these organisms is highly specialized for internal fertilization (see figure 5.24). Most species are viviparous. In addition, most species have varying degrees of placentation. Only the chimaeras (i.e., Raja) and a few families of sharks (e.g., Scyliorhinus) lay eggs. In male elasmobranchs, the sperm is collected by vasa efferentia which lead into the reproductive portion of the mesonephros. This coiled, thick-walled vas deferens secretes a material that aggregates the sperms into spermatophores. The spermatophores are then stored in a broader ampulla which leads to a "forwardly directed blind diverticulum," or sperm sac (12:137). Spermatophores are transmitted to the female by a large pair

Category: Science - V