OCR Text |
Show 1891.] YOUNG OE ECHIDNA ACULEATA. 11 this is much thicker than the non-sensory epithelium covering the shelf, which is columnar and stratified, and bears especially strong cilia (fig. 16). I was unable to recognize any cilia on the sensory epithelium. The function of " Jacobson's turbinal" must therefore be a purely mechanical, and not a sensory one. The subepithelial tissue on the dorsal and internal side of the organ encloses large bundles of olfactory nerves, which send branches to the other parts. A number of small gland-tubes are present in the connective tissue of the turbinal, and these open at intervals into the lumen of the organ. O n the dorsal and lateral side of Jacobson's cartilage a thick mass of tissue is preseut between it and the epithelium covering the ridge which projects into the nasal cavity and in which the whole organ is enclosed. This tissue contains a number of simple glands, which also extend posteriorly to Jacobson's organ, externally to the more complicated " septal gland," and open at intervals into the nasal cavity (figs. 6-11 and 14-16); the septal gland with its ducts is therefore of great extent, passing along almost the entire length of the nasal chamber. The epithelium covering the whole ridge is columnar and ciliated like that lining the general nasal cavity, and passes into stratified pavement-epithelium behind the naso-palatine duct, the latter form extending back still further on the nasal floor. Gland-tubes similar to those just described are also present in great abundance beneath the epithelium of the posterior part of the maxillo-turbinal (=Steno's gland?) (figs. 11 and 15), and also to a less extent beneath that of the ethmo-turbinals and other parts of the nasal cavity. I do not propose to describe the structure of the eye here, and will only mention that in the young Echidna it lies some distance from the surface, and a groove, lined by a thick cuticle, extends inwards towards it from the integument. In Stage I. the two layers of epithelium bounding this groove join at its base so as to form a solid band connecting the conjunctiva with the epidermis. In Stage II. the eyelids are beginning to separate, a narrow-slit being present in their middle part. There is a very large Harderian and a smaller lacrymal gland, and folds of the epithelium of the eyelids apparently represent the developing Meibomian glands. The naso-lacrymal duct (n.d.) opens into the outer side of the nasal chamber rather further back than Stensons duct, in a bay between the " ali-nasal turbinal" and the floor of the chamber. From this point it extends directly backwards, just outside the nasal capsule, to the conjunctival chamber. Neglecting their more obvious resemblances and differences, and confining ourselves to the observations recorded above, it will |