No. 2.] EMBRYOLOGY OF THE LIZARD. 333 



to form the vitreous humor; but this cannot be said to separate 

 the eye from the epiblast. A proximity to the epiblast is a 

 fundamental necessity for an organ of sight, and this has un- 

 doubtedly been one of the chief formative motives in the early 

 growth of the eye. Figs. 45 and 46, PI. XV., represent two early 

 stages of the growth of the eye, the sections being cut nearly 

 horizontal, and parallel to the longitudinal axis of the primary 

 fore-brain. In Fig. 45 the optic vesicle extends laterally, with a 

 slight posterior bend at the distal end. In Fig. 46 the greater 

 part of the vesicle extends in a posterior direction, so that what 

 is at first the posterior wall becomes later the median wall. This 

 wall may be distinguished from the anterior wall, as the postero- 

 median wall does not touch the epiblast. About this time the 

 distal expansion of the vesicle makes the stalk appear as though 

 dorso-ventrally constricted, the lumen being round. The 

 postero-median wall develops a heavy pigment on its inner sur- 

 face (toward the vesicle). If we presume the existence of an 

 ancestral form with eyes, the outer surface of whose head cor- 

 responded with the present inner surface of the brain, then these 

 pigmented spots would seem to indicate the position of those 

 primitive eye-spots, which, as the brain became enclosed by 

 the sinking in of the middle part of the head, first faced 

 anteriorly and then laterally. On this supposition, the anterior 

 parts of the brain-wall, thus brought between the primitive eye- 

 surface and the epiblast, would, being nearest the surface, 

 tend to assume the optic functions, and the primitive eye would 

 thus degenerate. This idea is suggested by the fact that in the 

 embryonic eye the anterior or external wall continues to grow 

 and differentiate, while the postero-median wall atrophies. The 

 general features of the development of the eye are so well known 

 that I will here refer only to a few points. An examination of 

 Figs. 45, 46, and series C and D, will show that the optic organs 

 are not ventral outgrowths, as they appear to be in the adult 

 brain, but are lateral outgrowths from the extreme anterior 

 region of the primary fore-brain, so that the anterior walls of the 

 optic stalks are in the same plane with the anterior surface of 

 the fore-brain. The optic vesicle becomes compressed by the 

 development of the lens, so that its walls touch, and it takes the 

 shape of a double-walled cup, with an unclosed slit (choroid slit) 

 at its anterior end immediately external to the distal end of the 



