Asexual reproduction is uncommon in animals; sexual reproduction is nearly universal and can be either hermaphroditic
or based on separate sexes. Embryonic development proceeds in an orderly fashion through early cleavages, blastula formation,
and (in most animals) gastrulation, neurulation, mesoderm formation, and organogenesis.
- REPRODUCTION
- Asexual reproduction occurs among hydras and sporadically across many phyla, but it is generally uncommon.
It provides no capacity for variation, except by mutation.
- A few species, like aphids (plant lice of the insect order Homoptera), can reproduce either sexually or asexually.
In the spring and summer, they reproduce several generations asexually, with all the offspring genetically identical to the "stem mother",
taking advantage of that year's climatic and other conditions. In the fall, they reproduce sexually and overwinter,
ready to emerge in the spring under unforseen and unpredictable conditions. The genetic variety produced by sexual reproduction
makes it more likely that at least some of them will be adapted to take advantage of the uncertain future conditions.
- Sexual reproduction is almost universal among animal species. It provides abundant variation among offspring.
- Hermaphroditism: each individual contains both male and female gonads, producing both eggs and sperm.
Most hermaphroditic species have adaptations that promote cross-fertilization and make self-fertilization difficult.
- Gonochorism (separate sexes): each individual is either male or female
EARLY STAGES IN HUMAN REPRODUCTION:
- Egg production takes place in the ovaries. The oviducts (or Fallopian tubes) carry the eggs to the uterus.
- Sperm production takes place in the testes. The vas deferens (also called ductus defens) carries the sperm
through the prostate gland into the urethra.
- Fertilization takes place inside the oviducts, followed by early cleavage stages.
- Implantation into the wall of the uterus usually occurs in the blastocyst stage, with about 64 cells.
Illustrations: Reproduction
- EMBRYONIC DEVELOPMENT
- Fertilized egg (zygote). The amount of yolk varies among animal groups.
- Early cleavages: One cell divides into two, then four, and so on. Eggs with little yolk or moderate amounts divide
completely, but those with large amounts of yolk divide only part way down, resulting in a disk-like embryo lying on top of the yolk.
- Blastula: usually a ball of cells with an internal cavity (blastocoel). Sponges go no further than this before becoming adults.
- Gastrula: a two- or three-layered embryo, formed by tucking-in of cells at one end to form a new cavity, the archenteron.
Cnidaria become adults at a two-layered gastrula stage of development. Most other animals develop a middle layer, or mesoderm.
- In the majority of animals, somites develop as segmentally organized blocks of mesoderm. Interior cavities within the somites
become the coelom.
- In the phylum Chordata, the ectoderm that lies above the notochord rolls up to form a tube, the neural tube, from which the
nervous system develops.
- Further development differs greatly among the different phyla and classes.
Illustrations: Embryological development
REVIEW:
Study guide and vocabulary
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