Meeting Abstract
The nasolacrimal duct (NLD) connects the orbital and nasal regions in many tetrapods. Caudally, this duct opens into the anterior/ medial orbital region, in close association with the nictitating membrane or an anterior orbital gland (e.g., Harderian gland: HG). The HG and NLD develop in close association with each other in several tetrapod vertebrates. In cetaceans, there is no NLD in the adult and the identity of the orbital glands is described differently by different authors, possibly reflecting taxonomic variation. Through examination of an embryological series of the pantropical spotted dolphin (Stenella attenuata) numerous divergences from other tetrapods were observed. Underdeveloped eyelids and a few ventral orbital glands were present by late Carnegie stage 17/18. By Carnegie stage 19, conjunctival glands encircled the eyeball. In fetal stage 20, these conjunctival glands proliferated, eyelids (and associated palpebral glands) formed, and a duct similar to the NLD appeared. There was subsequent reduction of palpebral glands and the NLD by fetal stage 21/22. There was regional growth of the conjunctival glands during these fetal stages. In contrast to other tetrapods, in which there are distinct zones of orbital gland development and the NLD connects the orbit to the nasal region before the lacrimal bone appears. This conjunctival ring of glands was described in some adult cetaceans and it was proposed to produce a protective oily sheet for the cornea. The development of both the NLD and orbital glands may be other examples of developmental drift. The fossil record shows that the NLD was present in the earliest, amphibious, freshwater cetaceans but was lost early on as they invaded the oceans.
