DEVELOPMENT OF THE EYE OF AMBLYOPSIS. 



149 



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 p. m. the embryos 

 had reached a length of 2.4 mm. (fig. 56 d). At 5 11 3o m 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 and connected only in front by the contracted optic stalk (fig. 

 57 a). 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 a. m. some of the embryos were still only 2.4 mm. long and 12 to 

 13 protovertebne had been formed (fig. 57 b). The changes in the eye from 12 

 p. m., May 5, to 12 noon, 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. 



57 MO- 



The changes from noon till 6 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 development of the olfactory pits. Seventeen protovertebrae had de- 

 veloped and the embryo was about 3 mm. long. 



Fig. 58. Horizontal Sections through Optic Stalk (tig. a) and Optic Vesicle , 



(fig. 6) of Embryo of Second Series. 



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 eggs in the early 

 stages of gastrulation. 



The eyes had reached a stage seen at about 2.5 to 3 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 considerably slower than 

 in the preceding series. Figure 57 e (March 13, 10 a. m.) was taken from a living 

 specimen, showing 5 protovertebrse. Sections demonstrated that at the stage repre- 

 sented by figure 57 e the neural tube was still a solid structure. The distance from 

 edge of eye to edge of eye measured 164 /x. 



About a day later the larvae were 2 mm. 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. 



