Gosner stage
Gosner stage refers to a generalized system describing stages of embryonal and larval development in anurans. The Gosner system includes 46 numbered stages, from fertilized embryo to the completion of metamorphosis. It was introduced by Kenneth Gosner in 1960. The system is widely used in herpetology to describe tadpoles. Gosner stages are based on certain morphological landmarks that allow comparing development in different species that may greatly differ in age or size.
Developmental stages
The Gosner system includes 46 stages, from fertilized embryo to the completion of metamorphosis. In the original publication, only two major developmental stages were distinguished, embryonic or prefeeding stages and larval stages. McDiarmid and Altig distinguish four major developmental stages, as detailed in the table below.Developmental category | Gosner stages | Description |
Embryo | 1 to 20 | Intracapsular. Development through cleavage, gastrulation, and appearance of neural tube and eventually gills and tail. |
Hatchling | 21 to 24 | Transition from relatively immobile embryo to an active, feeding tadpole. Specimens at these stages may sometimes be referred to as "larvae". |
Tadpole | 25 to 41 | Longest part of larval period that is marked by growth and development of limbs. The generic term "larvae" often refers to these stages. |
Metamorph | 42 to 46 | Loss of larval characters and development of adult structures; typically associated with the transition from the aquatic to the terrestrial habitats |
In ecological literature, the term "hatchling" may refer to stages 24–26. However, for direct-developing species, hatching occurs in the end of the development.