THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 173 



fertilization. The time when the eye begins to form is exactly as in fishes with 

 normal eyes. 



At 11.00 a.m., May 5, 1901, the head was slightly raised so that its outlines 

 appeared definite and clear, while the remaining outlines of the embryo were hazy. 

 It was not possible at that time to distinguish eyes (PL XIII, Fig. 13). At 2.30 p.m., 

 when the embryo has reached a length of 1.6 millimetres, the eyes form prominent 

 lobes on either side of the brain. The lobes are distinguishable in living embryos, but 

 stand out much more prominently in embryos mounted entire. In an embryo pre- 

 pared in this way, a camera outline of which is reproduced in Figure 14, the eye pro- 

 tuberance (oc.) has a length of 80 micra and projects 36 micra beyond the lateral 

 margin of the brain. Sections of embryos at this stage of development show the 

 brain to be still joined with the ectoderm. There is no indication of any cavity in the 

 central nervous system at this time, and the eye-lobes are solid, symmetrical lateral 

 protuberances with their anterior margins but 48 micra from the tip of the brain. 

 At 6.00 p.m. the embryo had reached a length of 1.76 millimetres and six protover- 

 tebrse had been formed. The eye was no longer a symmetrical swelling on the side 

 of the brain, but its outer, posterior angle was now distinctly farther back than the 

 posterior inner angle. In other words, the lobes had grown laterad and were bent 

 backward. The lateral projection of the eye beyond the contour of the brain amounts 

 to 48 micra and has a longitudinal extent of 100 micra. The greatest diameter — 

 measured from the anterior inner angle of the eye to the posterior outer — was 116 

 micra. Sections show the nervous system, including the eye, to be still a solid mass 

 of cells, which anteriorly is still continuous with the ectoderm. Histologically there 

 is no difference between the cells composing the optic lobes and those composing the 

 brain. There is a slight indication in the arrangement between the two optic lobes 

 suggesting a lateral traction of the cells (Fig. 15). At 9.00 p.m. the characters of 

 the eye shown at 6.00 p.m. had become intensified without other material change. 

 The embryo had reached a length of 1.92 millimetres and ten protovertebrse had 

 been formed. The optic lobe was still broadly united with the brain, but its lateral 

 growth was largely represented in the lobe extending back. There was no cavity 

 as yet in the nervous system. A little later the canal of the central nervous system 

 made its appearance, for at 12.00 p.m. it was well formed. There was probably some 

 fluctuation as to the rate of growth in length and the degree of differentiation the 

 tissues reach, for, in embryos of another series, some individuals had a well-developed 

 canal, while others of the same size did not. At 12.00 p.m. the embryos had reached 

 a length of 2.4 millimetres (Fig. 16). At 5.30 a.m., May 6, the eyes had become a 

 pair of flaps lying along the sides of the brain or diverging from near its anterior end 



