14.4: Fertilization - Biology

Haploid cells that must fuse together are called gametes. Gametes do not grow by mitosis, they must fuse to another haploid cell to produce a zygote, the first diploid cell in an organism. This process is called fertilization and it is composed of two distinct stages: plasmogamy and karyogamy. During plasmogamy, the cytoplasm of the two gametes combines together (plasm- referring to cytoplasm, -gamy meaning marriage). In most organisms, karyogamy is simultaneous with plasmogamy and the nuclei of each gamete fuse together into a single diploid nucleus. When you learn about fungi, you’ll see that plasmogamy and karyogamy can be separated by long periods, resulting in a strange condition called being dikaryotic (having two nuclei).

Each gamete produced during meiosis is genetically distinct, resulting in a pool of genetic options. During sex, organisms don’t choose which gametes are combined, so any combination of genes from the two parents is possible. This is called random fertilization and exponentially increases the potential for diversity within a population.

How many different zygotes could you make by combining the following two sets of gametes produced by meiosis?

Parent 1 eggsParent 2 sperm

Fertilization decreases plant biodiversity even when light is not limiting

Many researchers hypothesize that plant richness declines at high soil fertility (and high productivity) due to light limitation. We tested this hypothesis in an old-field by independently manipulating fertilization and light levels via shade cloth (decreased light), vegetation tie-backs (increased light) and vegetation clipping (increased light). Droughts occurred during two of the four years of the study, and we found that higher light levels were generally associated with decreased plant richness in drought years but increased plant richness in wet years. Most importantly, fertilization decreased richness whether light availability limited richness (wet years) or did not limit richness (drought years), and the effects of fertilization and light manipulation treatments were additive. These results suggest that effects of fertilization on plant richness are at least partly independent of light levels and that competition for resources other than light plays a substantial role in the decline of plant richness after fertilization.


  • Understand the mechanisms of gamete formation.
  • Understand the mechanisms of cell division.
  • Describe the differences between mitosis and meiosis.
  • Understand the mechanisms of fertilization, both in vivo and in vitro.
  • Describe the cleavage of the zygote.
  • Have a preliminary understanding of the role and process in male sex determination and X inactivation.
  • Understand the abnormalities that occur during this period of development.

14.3 Comparative embryology

Comparative embryology is the branch of embryology that compares and contrasts embryos of different species. It is used to show how all animals are related. Many things are compared (such as whether or not the organism has a notochord or gill arches). Many components go into comparative embryology, and much information about the developmental similarities between species can be taken from its study, from which many conclusions can be drawn. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor. The goal of comparative embryology is to make sense of how an embryo develops, and of how all animals are related. Comparative embryology also supports evolutionary theory, in the sense that all vertebrates develop similarly. The conclusion is that all vertebrates must have a common ancestor.


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Archosaurs: Pterosaurs

More than 200 species of pterosaurs have been described, and in their day, beginning about 230 million years ago, they were the undisputed rulers of the Mesozoic skies for over 170 million years. Recent fossils suggest that hundreds of pterosaur species may have lived during any given period, dividing up the environment much like birds do today. Pterosaurs came in amazing sizes and shapes, ranging in size from that of a small song bird to that of the enormous Quetzalcoatlus northropi, which stood nearly 6 meters (19 feet) high and had a wingspan of nearly 14 meters (40 feet). This monstrous pterosaur, named after the Aztec god Quetzalcoatl, the feathered flying serpent that contributed largely to the creation of humankind, may have been the largest flying animal that ever evolved!

Some male pterosaurs apparently had brightly colored crests that may have served in sexual displays some of these crests were much higher than the actual head! Pterosaurs had ultralight skeletons, with a pteroid bone, unique to pterosaurs, that strengthened the forewing membrane. Much of their wing span was exaggerated by a greatly elongated fourth finger that supported perhaps half of the wing. It is tempting to relate to them in terms of bird characteristics, but in reality, their proportions were decidedly not like birds at all. For example, it is common to find specimens, such as Quetzalcoatlus, with a head and neck region that together was three to four times as large as the torso. In addition, unlike the feathered bird wing, the reptilian wing had a layer of muscles, connective tissue, and blood vessels, all reinforced with a webbing of fibrous cords.

In contrast to the aerial pterosaurs, the dinosaurs were a diverse group of terrestrial reptiles with more than 1,000 species classified to date. Paleontologists continue to discover new species of dinosaurs. Some dinosaurs were quadrupeds ((Figure)) others were bipeds. Some were carnivorous, whereas others were herbivorous. Dinosaurs laid eggs, and a number of nests containing fossilized eggs, with intact embryos, have been found. It is not known with certainty whether dinosaurs were homeotherms or facultative endotherms. However, given that modern birds are endothermic, the dinosaurs that were the immediate ancestors to birds likely were endothermic as well. Some fossil evidence exists for dinosaurian parental care, and comparative biology supports this hypothesis since the archosaur birds and crocodilians both display extensive parental care.

Dinosaurs dominated the Mesozoic era, which was known as the “Age of Reptiles.” The dominance of dinosaurs lasted until the end of the Cretaceous, the last period of the Mesozoic era. The Cretaceous-Tertiary extinction resulted in the loss of most of the large-bodied animals of the Mesozoic era. Birds are the only living descendants of one of the major clades of theropod dinosaurs.

Watch the video: Pollination and Fertilization in plants. Science. Grade-3,4. TutWay (January 2022).