The ultrastructure of the cornea‐lens epidermis in the lateral eye of Limulus polyphemus

The structure of the “Corneagen,” i.e., the epidermis lying beneath the cornea‐lens of the lateral eyes of the adult intermolt Limulus polyphemus was studied with light and electron microscopy. This layer is composed of heavily columnar cells containing a striking number of cytoplasmic microtubules. Many of the microtubules are grouped into compact bundles or fascicles, generally each cell having at least one microtubule bundle. The cornealens end of each cell has numerous microvilli, each with a core of delicate filaments. The crypts between microvilli end in extracellular expansions and plaques of electron dense amorphous material are associated with these terminal expansions. Cytoplasmic microtubules appear to insert into these dense areas. The basal ends of the cells are thrown into many pseudopodial processes which extend into the surrounding extracellular space. The cytoplasm of the pseudopodia is composed largely of microtubules and their associated low density halos. Junctional complexes consisting of zonulae adhaerens and septate desmosomes are present between adjacent cells. Mitochondria, ER, cytoplasmic vesicles, Golgi stacks and other ultrastructural details of the epidermal cells are described. The ultrastructure of a column of pigment free processes lying between the apex of the lens cone and the underlying photoreceptive portion of the ommatidium is also described. Ducts or vessels of uncertain origin are present in the inter‐ommatidial spaces. Possible roles played by the microtubules, the significance of their disposition and of their association with the dense subsurface plaques are discussed in terms of intracellular support, epidermis‐lens attachment and extracellular pattern determination. In addition, the likelihood of the dense plaques being the site of microtubule assembly is considered.