Besides the retina, all eyes possess a dioptric apparatus. What is the evidence as to its nature in these vertebrate median eyes? Lankester and Bourne have divided the eyes of scorpions and Limulus into two kinds, monostichous and diplostichous. In the first the retinal cells are supposed to give rise to not only rhabdites but also the cuticular chitinous lens, so that the eye is one-layered; in the second the lens is formed by a well-marked hypodermal layer, in front of the retina, composed of elongated cells, so that these eyes are two-layered or diplostichous. The lateral eyes, according to them, are all monostichous, but the median eyes are diplostichous. [Illustration: FIG. 35.--EYE OF ACILIUS LARVÆ. (After PATTEN.) _l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._, rhabdites; _ret._, retinal end-cells.] This distinction is not considered valid by other observers. Thus, {84}as already indicated, Patten looks on all these eyes as three-layered, and states that in all cases a corneagen or vitreogen layer exists, which gives origin to the lens. For my own part I agree with Patten, but we are not concerned here with the lateral eyes. It is sufficient to note that all observers are agreed that the median eyes are characterized by this well-marked cell-layer, the so-called vitreous or corneagen layer of cells. [Illustration: FIG. 36.--EYE OF HYDROPHILUS LARVA, WITH THE PIGMENT OVER THE RETINAL END-CELLS. _l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._, rhabdites; _ret._, retinal end-cells.] This layer (_c._, Fig. 35) is composed of much-elongated cells of the hypodermal layer, in each of which the large nucleus is always situated towards the base of the cell. The space between it and the retina contains, according to Patten the cells of the pre-retinal layer _(pr.)_. These may be so few and insignificant as to give the impression that the vitreous layer is immediately adjacent to the retina (_ret._). Let us turn now to the right pineal eye of Ammocoetes (Fig. 37) and see what its further structure is. The anterior part of the eye is free from pigment, and is composed, as is seen in hæmatoxylin or carmine specimens, of an inner layer of nuclei which are frequently arranged in a wavy line. From this nucleated layer, strands of tissue, free from nuclei, pass to the anterior edge of the eye.