Bufo americanus kife cycle & development

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Life cycle from embryo to adult in Gosner stages

Life Cycle Stages

Many staging tables of anuran development from fertilized egg to fully metamorphosed individual were developed. Number of stages vary from 46 (Gosner) to 66 (Nieuwkoop and Faber). Here, generic developmental stages of bufonid (toads) and ranid (frogs) without precise timing are presented. The sequence of changes in the early embryo from fertilization through cleavage, blastula, and gastrula are, essentially similar in most species. Whenever possible, data from available literature about B. americanus were incorporated. Gosner (1960) staging table was used as a foundation. Photos of tadpoles on different stages of development (new window).

The total change from egg to small toad takes about 6 to 8 weeks.

• Oocyte MeSH Oocytes, precursors to mature eggs, are stored in the ovarian lobes of the female amphibian; oocytes are classified from stage I to stage VI; stage V and VI oocytes are used for electrophysiological experiments and are harvested by survival surgery or after euthanization of the frog
• Embryo MeSH Ferilized egg to tadpole; Gosner stages 1-46.
  • Ferilized egg Zygote, 1-cell embryo; 0-45 min post-fertilization; designated as stage 1 in all staging systems. After mating takes place, B. americanus females lay their eggs (4000 to 8000 in total) in the water, in long spiral tubes of jelly. Each individual egg is 1.5 mm in diameter and is black on top and white on the bottom.
  • Cleavage MeSH Cells divide rapidly without growth; 2-16 cells. The embryo consists of two "hemispheres": light (opposite to the point of sperm penetration) and dark.
  • Morula MeSH "Solid ball" consisting of 64 cells (blastomeres), cells start to differentiate.
  • Blastula MeSH Approximately 100,000 cells
  • Gastrula MeSH Approximately 30,000 cells; in this stage, the cells rearrange themselves and three germ layers are formed: the ectoderm, the endoderm and the mesoderm.
  • Organogenesis Formation and development of the organs.
    • Neurula
    • Tailbud
• Tadpole Larval stage
  • Hatching Embryos of most species hatch between Gosner stages 17 and 20, where stage 18 is recognized by the initiation of muscular flexures and stage 19 by the heart beat. Full development of external gills is completed between stages 21 and 23.
  • Early tadpole Adhesive organs (oral suckers) are developed. Larvae did not start feeding and are not yet free-swimming.
  • Mid-tadpole Feeding and free-swimming larvae.
    • Gosner stage 21 The cornea become transparent and the eyes are clearly visible; tail fins are still opaque.
    • Gosner stage 22 The tail fins become transparent.
    • Gosner stage 23 External gills disappear as they become covered and hidden by the backward growth of a shield-like skin fold called operculum. Initial formation of pigmentary patterns, chromatophores (melanophores, lipophores, and iridophores) begins.
    • Gosner stage 24-25 From stage 25 on, a sinistral-single, lateral spiracle (the external opening for exit of water from the opercular chamber) on left side of body is present on the left side of most North American tadpoles. Independent feeding begins.
    • Gosner stage 26 The oral disc and labial tooth rows that started to form at stage 23 may be used for family identification at this stage. Limb buds start to develop.
    • Gosner stage 27-30 Elongation of limb buds gradually progresses.
• Late tadpole Beginning of more drastic changes in the course of metamorphosis.
  • Gosner stage 31 The "foot" is paddle-shaped.
  • Gosner stage 32-39 Differentiation of toes and proportional changes in the length of individual toes and appearance of metatarsal and subarticular tubercles.
  • Gosner stage 40-41 Resorption of the tail begins. Mouth parts begin to break down. Skin over the forelimbs becomes transparent.
  • Gosner stage 42-46 Appearance of forelimbs. Metamorphosis of the head, particularly, mouth.
• Toadlet Fully transformed young may or may not resemble the adults sufficiently to permit identification. Newly-metamorphosed toads stay near their pond for a few days, and then disperse.
• Adult Sexual maturiry can be attained at 1 year of age (next year following metamorphosis). However, reproductive development in toads living at high population densities may be delayed for several years. Most breeding female American toads are 2 - 5 years old. Average length of adult sexually mature toad is about 75 mm (2.95 in.) ranging from 50 to 102 mm (1.97 to 4.02 in) in length. Body weight ranges from 14 to 48 g. Females are usually larger than males.

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