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 2.8 mm. long of first series. 



During the next 24 hours the embryo grew to a length of 2.4 mm. At this 

 stage the tail was free for 0.4 mm. of its length. Embryos 24 hours older than the 

 last were found to be 2.5 to 2.8 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, r) 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 



He. 50. (a) Horizontal Section of Head of Embryo 2.5 mm. long, two Sides at Different Levels. 



(6) Left Eye of Same Embryo as that from which lig. 5g a was taken, showing First Indication of Lens. 



(c) Transverse Section through Dorsal Part of Optic Stalk of Embryo 2.7 mm. long. 



(d) Optic Vesicle and beginning of Lens in another Specimen 2.7 mm. long. 



(c) Transverse Section of Optic Vesicle and beginning of Eye of a Cymatogaslcr larva, 1.5 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 Cym/it/i^istcr 1 larva about half as long. (Com- 

 pare figs. 59 r, <7.) 



There is no indication of a differentiation of an iris. The secondary cup is a 

 -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' J). 



FOUR-MILLIMETER STAGES. 



In specimens 4.4 mm. long the eye had become a deeper cup than it was during 

 The lens, which no longer fills the entire cavity, has become 



with l.irnc an. I <:-]|-ili-vdopcrl (.-yes. Figures 60 a, b (Cvmatogasler) should lie 



