ACTIVITY 2B: ANIMAL DEVELOPMENT

• Development of organisms may be of two types: phylogeny and ontogeny.
• Phylogenetic development refers to the evolutionary history of the species while ontogenetic development refers to the entire life history of an organism.
• Fertilization involves the union of the sperm and egg nuclei. This process restores the chromosome number or diploid number of the organism.
• The embryo as a result of fertilization is appropriately called a zygote.
• Cleavage is the process of transforming the single celled zygote into a multicellular entity. This process involves a series of mitotic division, which transforms the zygote to several cleavage cells or blastomeres.
• Blastulation is the process that involves an internal secretion of fluid by the blastomeres of the morula. This process results in the formation of a fluid-filled cavity, the blastocoel or segmentation cavity. The embryo at this stage is referred to as blastula.

  • In addition to the formation of the cavity is the emergence of distinctly two populations of cells.
  • The layer of cells immediately surrounding the blastocoel forms the endoderm while the outer layer of cells forms the ectoderm. At this stage, the embryo is characterized as didermic (di= two, dermis= layer/coat/skin).
• Gastrulation is the stage that involves major cell movements and rearrangements to form the body plan characteristic of a species. These phenomena are collectively referred to as “morphogenetic movements”.
• Accomplished during gastrulation (gaster= stomach) is the laying out of the primitive gut, also known as gastrocoel or archenteron. This is the future digestive tract of the embryo. An opening leading to the gastrocoel called blastopore is formed in the earlier part of the gastrula stage. It is the future anal or cloacal opening in deuterostomes (echinoderms and vertebrates). In protostomes such as nematodes, mollusks and arthropods, the blastopore becomes the mouth.
• Another important event of gastrulation is the establishment of the three germ layers namely, ectoderm, mesoderm, and endoderm. These germ layers serve as the foundation for the organ systems of the animal to take shape. The table below provide examples of organ system and their corresponding germ layer origin.
• Once the germ layers are established, the different organs start to form. This process is called organogenesis.
• One of the organ systems that take shape the earliest is the central nervous system. This system of vertebrates develops from a dorsally situated ectodermal thickening called neural plate.

  • This structure invaginates to form a neural fold and a depression called neural groove, which eventually develops into a hollow neural tube.
• The process involving the development of the CNS is called neurulation and the embryo at such stage is referred to as neurula.
• The other organ systems form during the neurulation stage.
• The frog’s egg is of mesolecithal type, which means it contains moderate amount of yolk. The stored food is depleted as development continues.
• The developing young emerge as a swimming larva called tadpole.

  • Organogenesis and histogenesis proceed during the tadpole stage
• Segmented or Unfertilized Egg:

  • The yolky material is not evenly distributed. It is concentrated in one area of the egg.
  • The area where the yolky material is concentrated is the vegetal pole.
  • The area opposite the vegetal pole is called the animal pole.
  • Such type of egg based on yolk distribution is referred to as telolecithal type.
• Cleavage:

  • Cleavage cells are also called blastomeres
  • The outlines or boundaries of the blastomeres can be seen with the furrow or space in between them
  • The blastomeres that are smaller are sometimes called micromeres and are situated at the animal pole.
  • The blastomeres that are larger are called macromeres and are situated at the vegetal pole.
• Blastula:

  • The fluid filled cavity found within the embryo. This is the blastocoel or segmentation cavity.
• Gastrula:

  • the prominent cavity, the primitive gut or archenteron. Find the opening leading to this cavity called the blastopore. At this stage, notice that the blastopore is filled with yolk plug. The blastopore is bounded dorsally by the dorsal tip of the blastopore and ventrally by the ventral lip of the blastopore
  • the neural folds meet at the center and move inward to form a neural tube. The neural tube serves as the precursor of the brain and spinal cord.
• The outer layer of the embryo is the ectoderm. The inner layer immediately surrounding the archenteron is the endoderm. The middle layer between the ectoderm and the endoderm is the mesoderm.
• This is the archenteron or the primitive gut. The lining of the gut is the endoderm.
• Mesoderm:

  • divided into segments or somites. The most dorsal somite is the epimere.
  • situated at the level of the notochord. The middle portion of the mesoderm forms a string of cells, which is the mesomere. T
• The mesomere is the origin of the urogenital system.
• The mesoderm extends ventrally towards the guts.
• The ventral most portion of the mesoderm is the hypomere or lateral plate mesoderm. This portion of the mesoderm is subdivided into inner splanchnic layer and an outer somatic layer.
• A cavity is formed between the splanchnic layer and the outer somatic layers, is the future coelom or body cavity of the animal.