174 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



and connected only in front by the contracted optic stalk (Fig. 17). The split in the 

 optic lobe which separates it into an outer and an inner layer had developed to such 

 an extent that it could readily be made out in living embryos. At 8.00 a.m. some 

 of the embryos were still but 2.4 millimetres long and twelve to thirteen protover- 

 tebrse had been formed (Fig. 18). The changes in the eye from 12.00 p.m., May 5, 

 to 12.00 M., May 6, were not very great, and consisted chiefly in the constriction of 

 the optic stalk and the consequent gradual separation of the optic lobe from the brain. 

 The skin had not yet begun to thicken to form the lens (Figs. 19, 20). The changes 

 from noon till 6.00 p.m., May 6, when the last embryo of this series was preserved, 

 consisted largely in the shifting of the optic vesicles as the result of the develop- 

 ment of the olfactory pits. Seventeen protovertebrse had developed and the embryo 

 was about 3 millimetres long. 



For later stages I am compelled to draw on another series of embryos which I 

 also observed through the earlier stages described above. They were taken from a 

 female that was captured March 11, 1898, and that contained on March 12 eggs in 

 the early stages of gastrulation. 



The eyes had reached a stage seen at about two and a half to three days from 

 the beginning of development. An outline of the development may be given to 

 connect this series with that just described. The rate of development was consid- 

 erably slower than in the preceding series. Figure 21 (March 13, 10 a.m.), was taken 

 from a living specimen, showing five protovertebrse . Sections demonstrated that at 

 the stage represented by Figure 21 the neural tube was still a solid structure. The 

 distance from edge of eye to edge of eye measured 164 micra. 



About one day later the larvae were 2 millimetres long. The neural canal had 

 been formed and extended out into the now well-formed vesicle through a distinct 

 optic stalk. Sections showed that the epidermis was still unmodified over the eye, 

 with no indication of a thickening to form the lens. 



Figures 22 and 23 (PI. XIV) show horizontal sections through the base of the 

 optic stalk and through the middle of the optic vesicle respectively. 



During the next twenty-four hours the embryo grew to a length of 2.4 millimetres. 

 At this stage the tail was free for .4 millimetres of its length. Embryos twenty-four 

 hours older than the last were found to be 2.5 to 2.8 millimetres 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. 24, 25, 

 26, 27) the eyes have become greatly modified. The secondary optic vesicle has 

 been formed by the thickening 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 



