THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 185 



i. The First Period. — During the first period the eye arises as a solid outgrowth 

 from the solid central nervous system when the embryo is about 1.5 millimetres long. 

 The outgrowth increases rapidly in size during the next .5 millimetre of growth in 

 length. The solid lateral outgrowth is bent back along the side of the brain, and its 

 connection with the brain becomes constricted into the optic stalk. A cavity approx- 

 imately central arises in the optic lobe at the same time that a cavity appears in the 

 central nervous system, which occurs when the embryo is about 2 millimetres in 

 length. The two layers of the optic vesicle formed by the appearance of the cavity 

 are of about equal thickness. A little later the secondary optic vesicle is formed by 

 the thickening of the skin over the eye and the consequent cupping of the distal face 

 of the eye. The process reaches its culmination when the embryo has a length of 

 4.4 millimetres. The lens is still connected with the skin and the two layers of the 

 secondary vesicle have become very different, the proximal one being one-layered, 

 the distal one several-layered. The details of the changes of this period have been 

 given in the preceding pages. 



At any time up to this length the eye might, as far as its structure is concerned, 

 give rise to a perfect eye in the adult. The eye so far follows phylogenic paths with 

 the reservation that no adult ancestor is supposed to have had eyes like these embry- 

 onic stages. 



2. The Second Period. — The development during the second period is direct and 

 leads to the condition obtaining at the end of that period. Some of the processes 

 are palingenic, some are of purely ontogenic significance, while still others (if I may 

 make the distinction) are degenerative. 



The optic nerve develops at the beginning of the period in an undoubted phylo- 

 genic way. As in the case of the eye as a whole, the nerve develops directly into its 

 full size. The details of its history are given under the head of the optic nerve. The 

 latter half of the history of the lens belongs entirely to this period. Its history is also 

 given under another head. The changes the lens undergoes during this period are 

 all katagenic, and some time before this period closes the lens has disappeared. 



The direct development of the optic vesicle of the beginning of this period into 

 the eye as found at the end of this period is very difficult to interpret satisfactorily. 



A comparatively very narrow marginal part of the secondary optic vesicle is 

 converted into the epithelial part of the iris. The lens is almost always entirely 

 excluded from the optic cup when the iris develops. The extreme shallowness of the 

 optic cup and the comparative thickness of the retina would lead one to expect the 

 choroid fissure proper to be a very short structure. The shallow cup develops into 

 the adult eye by processes like those that take place in normal eyes. These purely 



