The nasal apparatus of the red squirrel (Sciurus vulgaris L.) embryo at the stage of the fully formed chondrocranium.

The nasal apparatus of the squirrel embryo at the optimum stage of the chondrocranium displays simple (though not always primitive) features and individual structures developed largely as in other rodents and even in insectivores. Primitive features include the presence of a cartilago paraseptalis communis and probably the simplicity of the olfactory labyrinth, whose main support in the region as a whole is ethmoturbinale I, whose dorsal and ventral lamella divide off the basic recesses; anterior (frontalis), maxillaris and frontoturbinalis. Completely caudally we find ethmoturbinale I, the frontoturbinalia and the relevant secondary (greatly reduced) recesses. The zona annularis is interrupted by failure of the rostral processes of the lamina transversalis anterior (corresponding to the processes laterales ventrales) to fuse with the capsula nasi ant. or with the anlage of the septum nasi. We did find a fenestra nasi superior (lateralis). The atrioturbinale is well developed, the maxilloturbinale only as a trace, but we have a very striking nasoturbinale. In the rostral region of the olfactory segment, the cartilaginous capsule has a very conspicuous thickening, which in this part is formed of paranasal cartilage. The thickening presents outwardly as the prominentia anterior; cavity of the recessus anterior (frontalis) is formed inside it. The rest of the dorsal and caudal wall of this cavity forms the crista semicircularis, which further caudally circumscribes the foramen olfactorium. A foramen epiphaniale is present. The vestibular region of the epithelial nasal tube is interestingly formed. In cross section it is crescent-shaped; the nasal tube itself opens into the convexity, but the lamina transversalis ant. sends a turbinale into the concavity. A cartilago alaris superior, which develops independently in situ, is present; in our stage it is associated with the wall of the capsule, but ventrolaterally it terminates freely and is not joined to any other element. The morphological interpretation of particular elements is discussed.