Naturally occurring diblastodermic eggs in the zebrafish, Brachydanio rerio

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Abstract
Naturally occurring monoblastodermic and diblastodermic eggs were obtained from one pair of mature zebrafish, Brachydanio rerio (Cyprinidae). The diblastodermic condition reported is unique for teleosts in that it is the first occurrence to be adequately documented in the literature. The formation of two blastoderms is due to a primary fission of the first two blastomeres involving an underlying aberrant egg organization. The subsequent formation of conjoined twins is attributed to secondary fusion of the two adjacent blastoderms during epiboly. Of 21 diblastodermic eggs laid in four separate spawnings, 9 died during epibolic overgrowth, 7 exhibited regulation and developed into normal or near-normal embryos, and 5 gave rise to duplicate embryos of the anadidymus type. The report describes and discusses this extremely rare mode of pregastrula twinning in bony fishes.

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