BI 102 Davison Lecture Notes, Spring 2003 

Ch 50. Animal Reproduction. 

Asexual and Sexual modes of reproduction are common among animals.  Asexual reproduction produces genetic clones and requires the cell division process of mitosis.  Asexual reproduction can be an effective, beneficial strategy for success in stable environments.  Sexual reproduction requires the cell division process of meiosis that occurs in gonads (ovaries and testes) followed by fertilization.  Some animals possess both ovaries and testes and are said to be hermaphroditic.  Even so, hermaphrodites such as earthworms, flatworms, and hydra usually do not self fertilize.  The combined result of meiosis (genetic segregation) followed by fertilization (genetic recombination) is greater variation among the offspring than possessed by the parents.  Sexual reproduction can be an effective, beneficial strategy for success during times of environmental change. 

Types of Asexual Reproduction 

  • Fragmentation followed by regeneration of lost body parts (e.g. aquatic annelids, planarians).
  • Budding – a new bud growth begins to generate an entirely new individual while attached to the asexual parent.
  • Parthenogenesis – requires the female organs of sexual reproduction (gonads and reproductive tract) but no fertilization ever occurs.  The micro-aquatic rotifers are normally reproduce by females producing diploid eggs (genetic clones of themselves) which develop into young individuals.  Aphids, a type of insect that feeds on plant sap, also reproduce via parthenogenesis in the summer.

Sexual Reproduction 

Both the events of meiosis and fertilization are common to all sexual reproduction.Internal Fertilization Requires Copulatory Organs.  Copulation is the physical union between a male and a female’s reproductive parts.  It may or may not result in successful fertilization.

  • The penis and vagina are the copulatory organs of mammals.
  • The cloaca (a common chamber through which digestive wastes, urine, and eggs pass) is the copulatory organ of most birds.
  • Male crayfish use modified swimmerets located below the abdomen.
  • A male octopus uses an arm (tentacle) to transfer a sperm packet to the female opening.
  • Male spiders employ pedipalps like syringes.  Pedipalps, frontal appendages that look like a 5th pair of legs, are used as suction reservoirs for their own sperm which was deposited on a web.  Once inside the bulbous segment of the pedipalp, the male spider is ready to court the female.  After successful contact with a female, male spiders use their pedipalps to inject the female with their sperm.  The image below shows the pointed apex of a male spider’s pedipalp.

External Fertilization occurs in animals that reproduce in water.

Egg and sperm are released into a watery environment at nearly the same time.  Fertilization takes place outside of the female’ body.  Ex. Spawning fish and frogs, echinoderms, cnidarians.  Earthworms have a modified external fertilization.  After copulating during which each hermaphroditic earthworm exchanges sperm with another, earthworms create a cocoon external to their clitellum.  Eggs from their own ovaries and sperm from their mate are deposited into the cocoon during cocoon formation.

Ch 51 Animal Development

Following live birth (viviparous) or hatching from eggs laid externally (oviparous) young animals must grow and change to become adults.  In some animals, such as insects and frogs, the changes are often dramatic and abrupt.  Such abrupt change in form as illustrated by the young caterpillar changing into the mature butterfly, or the wiggly tadpole changing into the warty toad is called metamorphosis.

Insects with complete metamorphosis go through four major stages:  egg, larva, pupa, adult.

Insects and other animals with simple or incomplete metamorphosis go through three major stages: egg, larva (the larval stage is sometimes called a nymph in insects), adult.  Vertebrates complete most of their transformation in form during embryonic development. 

Embryonic Development of Animals, especially vertebrates.

Knowledge of embryonic development is useful in animal classification and understanding evolutionary history.  A generalized account of animal embryo development is given on p. 920-921 in Mader and in the three-page handout given to you.  You should know the embryonic development differences as presented between protostomes (mollusks, annelids, & arthropods) and deuterostomes (echinoderms and chordates).  Know that gastrulation is a period of cell migration resulting in the formation of embryonic germ layers.  In cnidarians only two gem layers and an incomplete digestive tract are formed.  In all other animals (except sponges) three germ layers are formed.  These germ layers (ectoderm, endoderm, and mesoderm) are primary tissues that eventually become the organs of the body.  Ectoderm eventually differentiates into outer skin, nervous system, and the lining of the mouth and rectum.  Endoderm lines the lumens of the gastrointestinal tract (esophagus, stomach, intestines) and forms the inner lining of the lungs.  Mesoderm forms muscle and bone as well as epithelial layers covering the outer portion of internal organs. 

Comparison of Vertebrate Embryos

Most fish and amphibian embryos are surrounded by a jelly-like coating that allows for rapid gas exchange and diffusion of metabolic wastes (e.g. ammonia) directly into the watery environment of pond or stream.  These embryos also possess a single extraembryonic membrane, the yolk sac that contains stored food for the developing embryo.  Birds, Reptiles, and Mammals (so-called amniotes) are fully terrestrial in regards to their embryonic development due to the presence of additional extraembryonic membranes.

Each of the additional extraembryonic membranes found in amniotes (chorion, amnion, & allantois, see fig. 51.11) can be understood as adaptations for embryonic development in a terrestrial environment.   Remember these points:  Fish and frog eggs laid in a pond are surrounded by large amounts of water.  The surrounding water allows for both gas exchange and removal of metabolic wastes. Finally, remember that all cellular life is bathed by a watery fluid and that the period of embryonic development for all animals must occur in complete submersion of water. 

  • The chorion in bird and reptiles interfaces with the inner, hardened eggshell and actively functions for gas exchange.  The chorion in placental mammals also functions for gas exchange but in these mammals the chorion interfaces with the womb not the eggshell.  Chorionic villi grow to increase the surface area for exchange between the embryo and mother.  The placenta consist in part of the chorion and in part of maternal tissue. 
  • The allantois in shelled eggs is the repository for metabolic wastes, especially nitrogenous waste in the solid form of uric acid.  In mammals, the allantois becomes part of the umbilical cord as does the yolk sac.  The placenta delivers needed food and takes away metabolic wastes.  So, in placental mammals the extraembryonic membranes yolk sac and allantois, are greatly modified to form the umbilical cord.
  • The amnion is a fluid filled sac that cushions and surrounds the developing embryo and fetus with life giving water.  Just prior to birth, most of you are familiar with the phrase “did her water break.”  This water breakage is due to the rupture of the amnion.  Following water breakage, it is soon time for the baby to breath air!  What an abrupt transformation in habitat.