. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. 150 BLIND VERTEBRATES AND THEIR EYES. Figures 58 a and b show horizontal sections through the base of the optic stalk and through the middle of the optic vesicle respectively. The embryo is 2 mm. long and in about the same stage of development as those mm. long of first series. During the next 24 hours the embryo grew to a length of mm. At this stage the tail was free for mm. of its length Embryos 24 hours older than the last were found to be to mm. in length. The latter, while not
. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Heteropygii. 150 BLIND VERTEBRATES AND THEIR EYES. Figures 58 a and b show horizontal sections through the base of the optic stalk and through the middle of the optic vesicle respectively. The embryo is 2 mm. long and in about the same stage of development as those mm. long of first series. During the next 24 hours the embryo grew to a length of mm. At this stage the tail was free for mm. of its length Embryos 24 hours older than the last were found to be to mm. in length. The latter, while not longer than the oldest embryos of the first series described, are evidently farther along in the development of the eyes. In all of these specimens (figs. 59 a, c) the eyes have become greatly modified. The secondary optic vesicle has been formed by the thick- ening of the skin to form the lens. The retinal wall of the vesicle is three series of cells deep, while the wall destined to form the pigment epithelium has become. Hg. so. (a) Horizontal Section of Head of Embryo mm. long, two Sides at Different Levels. (b) Left Eye of Same Embryo as that from which fig. 59 a was taken, showing First Indication of Lens. (c) Transverse Section through Dorsal Part of Optic Stalk of Embryo mm. long. (d) Optic Vesicle and beginning of Lens in another Specimen mm. long. (c) Transverse Section of Optic Vesicle and beginning of Eye of a Cymatogasler larva, mm. long. thin and is composed of a single series of cells. The eye, at this stage, does not differ materially from that of a Cymatogasler1 larva about half as long. (Com- pare figs. 59 c, d.) There is no indication of a differentiation of an iris. The secondary cup is a shallow, bowl-shaped structure, the depression being entirely filled by the thicken- ing of the skin which is giving rise to the lens (figs. 59 b and d). FOUR-MILLIMETER STAGES. In specimens mm. long the eye had become a deeper cup than it was during the
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