148 



BLIND VERTEBRATES AND THEIR EYES. 



3 days from fertilization. The degree of development when the eye begins to form 

 is exactly as in fishes with normal eyes. 



At ii 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 (fig. 56 a). At 2 h 30™ p. m., when the 

 embryo has reached a length of 1.6 mm., 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 prepared in 

 this way, a camera outline of which is reproduced (fig. 56 b), the eye protuberance 

 (oc.) has a length of 80 fi and projects 36 //. 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 /u. from the tip of the brain. At 6 p. m. 

 the embryo had reached a length of 1.76 mm. and 6 protovertebrse 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 pos- 

 terior 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 /a and has a longitudinal extent of 100 /x (fig. 56 c). The greatest 

 diameter — measured from the anterior inner angle of the eye to the posterior 

 outer — was 116 fj.. 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 differ- 

 ence between the cells com- 

 posing 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. At 

 g p. m. the characters of the 

 eye shown at 6 p. m. had be- 

 come intensified without other 

 material change. The embryo 

 had reached a length of 1.92 

 mm. and 10 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 p. m. it was well formed. There was probably 

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



Fig. 57. 

 (d) Outline of Brain and Optic Vesicle of Living Embryo between sizes 



of those shown in tigs. 56 d and 57 b (^.50 a.m.. May 6). 

 (6) Outline of Brain and Optic Vesicle of Living Embryo 2.4 mm. long, 



with 12 or is Protovertebne (8 a.m.. May 6). 



(c) Horizontal Section through Left Eye of Embryo about 2.44 ram. 

 long, 2 Sections Ventrad of one represented in fig. 56 d. 



(d) Horizontal Section through Head of Same Individual, showing Optic 

 Vesicle (11 a.m., May 6). 



(<0 Outline of Brain and Optic Vesicle of Embryo 1.68 mm. long, with 5 

 Protovertebrse from Living Specimen. 



