DEVELOPMENT OF THE EXTERNAL NOSTRILS AND CHOANAE IN LACERTA AGILIS (REPTILIA, SQUAMATA)

In view of the transition to true terrestrial mode of existence in the olfactory organ of modern reptiles a clear differentiation between respiratory and olfactory parts took place, concha and nasopharyngeal canal appeared. The objective of this work is to show the particularities of morphogenesis of the external nostrils and choanae at successive stages of the development Sand lizard. Stages of ontogenesis were determined according to the tables of normal development for Lacerta vivipara. On the whole 12 embryos from 28th to 37th stage of Lacerta agilis were studied (Linnaeus, 1758). Dehydration, paraffin embedding, cutting of the material into 5-7 µm and 10-12 µm in frontal and transverse plane were carried out according to standard histological techniques. Preparations were stained with hematoxylin-eosin and by Steedman's method. Photos of histological sections were obtained using a microscope Zeiss Axio Imager M1. Due to the dorsal invagination of placodes at the 28th stage of development the olfactory pits are shaping. At the 29th stage the olfactory pits turn into olfactory sacs that open inwards stomodeum. 32nd stage is characterized by the fact that two separate openings develop into a rostral one (primordium of external nostrils) as well as into a caudal one (primordium of internal nostrils, or choanae). Olfactory sacs are transformed into the nasal cavity. At the 33d stage external nostrils diminish, get a slit-like shape and shift rostrally from choanae. At the 34th stage external nostrils open on the lateral surface of the head. Rostral part of the olfactory organ forms the vestibulum. At the stage of 35-37 external nostrils shift more laterally, increase in size and at the 37th stage they get the definitive topography. Choanae stretch and become slit-like. Our research has shown that the olfactory organ undergoes a series of successive stages of development of: olfactory placodes – olfactory pits – olfactory sacs – nasal cavity. Owing to the shaping of the outgrowth at the 32nd stage the opening of olfactory sac bifurcates into the external nostrils and choanae that are next to each other. Slabý O. in his research also points out these morphological rearrangements. Differences in the development of reptiles' structures may be associated with morphological reorganizations in the olfactory organ which are related to separation of vomeronasal organ and to shaping its own canal opening in the mouth. Thus, the shaping of external nostrils and choanae in L. agilis takes place through the formation of the septum in the opening of olfactory sacs. These morphological changes lead to the formation of functionally formed nasal cavity. After their formation the external nostrils shift rostrolaterally and get narrow. By contrast, choanae increase and become slit-like.