The present study analyses the sequence of metatarsal and phalangeal bone formation in Podocnemis expansa. The ontogenetic sequence studied comprises 24 embryos collected at 5-days intervals in various stages of prenatal development, starting from the 18th day of natural incubation. Biometric measurements were taken and the embryos were cleared and stained with alizarine for bone visualization, followed by the Davis and Gore method. Each pe has five metatarsals (M) and 14 phalanges, two on the fifth finger and three on each of the other fingers, with a phalangeal formula of 3:3:3:3:2. Retention of the stain in metatarsals occurs in the following order: M III > M II = M IV > M V > M I. In stage 20, it is evident that the ossification center progresses in the direction of the epiphyses in all metatarsals. The sequence of stain retention in distal phalanges (DP) occurs in the following order: DP V > DP IV > DP III > DP II > DP I. In the medial phalanges (MP), the stain retention sequence is MP III > MP IV > MP II > MP I. Lastly, the proximal phalanges (PP) retain staining in the following sequence: PP V > PP I > PP II > PP III > PP IV. Differences and similarities in synchronization of ossification in Podocnemis expansa and comparable species are evident. The lack of a common osteogenetic pattern among all chelonians is due to variations in the initial site of bone formation and sequence of ossification.
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Vol. 2 • No. 1