No. 2.] EMBRYOLOGY OF THE LIZARD. 357 



light. The lens being developed in the epiblast would do away 

 with a lenticular growth in the outer anterior wall of the vesicle. 

 The outer anterior wall, as has been stated, is covered externally 

 with fibres. If, going a step farther, we suppose the cellular 

 elements of the outer anterior wall, being nearer the source of 

 light, to have gradually assumed the optic function, we should 

 then have an eye in which the nerve-fibres lay between the cell- 

 ular elements and the light. The posterior median wall of the 

 vesicle, having become useless, would degenerate, together with 

 its nerve-fibres and the fibres connecting it with the brain along 

 the posterior wall of the optic stalk. On the other hand, that 

 part of the longitudinal fibres supposed to have been present on 

 the outer anterior wall of the vesicle would remain as the optic 

 nerve, and unite with the brain and anterior band of fibres along 

 the anterior wall of the optic stalk. The origin of the remark- 

 able conditions peculiar to the eyes of vertebrates is an ex- 

 tremely difficult subject to explain. The above explanation 

 seems to me to accord best with the main facts, as I have found 

 them in the lizard. 



The cranial flexure is an almost universal feature in vertebrate 

 embryos, and is of a nature so striking that it must have attracted 

 the attention of all embryologists. One explanation of this 

 feature generally given is, that it is due to unequal growth in the 

 dorsal and ventral halves of the brain. But it remains a mystery 

 what purpose this unequal growth subserves, and why it should 

 cause such a peculiar curve in the entire anterior end of the em- 

 bryo, — a curve which must later be rectified in part by the 

 secondary flexure. I have remarked upon and figured certain 

 transverse folds in the floor of the brain, where the flexure is 

 greatest, which discountenance this theory as the sole explana- 

 tion of the facts. Another explanation is, that the early and 

 great development of the brain makes it necessary that it should 

 partly roll itself together in order to accommodate itself to the 

 space allowed it for storage. This seems hardly to explain why 

 the curve should be almost universally of the same character, 

 or why it should bend out of line the notochord and intestine. 

 It is, moreover, improbable that a feature so fundamental in de- 

 termining the shape of the adult brain and head should have 

 been acquired simply to accommodate the wants of an early em- 

 bryonic condition. The lateral twist of the head may be ac- 



