Variation in the formation of crista sellaris and basisphenoid in the skull of the grass snake Natrix natrix embryos (Serpentes, Colubridae).

The crista sellaris is one of the basic structures of the snake chondrocranium. In embryogenesis it ossifies, forming the basisphenoid. This article describes variations of the crista sellaris, which, in turn, leads to variation in the formation of the basisphenoid in the skull of grass snake Natrix natrix (Serpentes, Colubridae) embryos. Several embryos at different developmental stages are investigated. Embryos from one developmental stage are taken simultaneously from one clutch. Thus, variation of the crista sellaris and basisphenoid in the skull of embryos is discovered, including those from one clutch. In several embryos, the crista sellaris is absent or partially formed. The absence of the crista sellaris does not preclude the formation of the basisphenoid. Based on the observed variations of the crista sellaris, the basisphenoid may have a different origin. In embryos with formed crista sellaris, the basisphenoid develops as a complex endochondral bone from three ossification centers (paired-in the polar cartilages and unpaired in the crista sellaris). In embryos without fully formed crista sellaris the basisphenoid develops as a paired bone of mixed origin: endochondral ossification appears in the polar cartilages, but where the crista sellaris is absent, it is formed by dermal ossification. In general, we observed the absence of the crista sellaris in 15% of studied embryos. We assume that it may be due to a reduction of the orbital-temporal region of the chondrocranium in N. natrix embryos. RESEARCH HIGHLIGHTS: The variation of the crista sellaris and basisphenoid formation in grass snake embryos is revealed. Depending on the presence or absence of the crista sellaris, basisphenoid is formed as a complex chondral bone or as a paired bone of mixed origin.