500 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [46] 



the fourteenth day as shown in Fig. 31. The rim of the cup seems, in 

 fact, to grow down from above and, gradually becoming more globular 

 in form as a whole, its free lower borders approximate and coalesce, 

 forming the choroidal fissure /c, through the proximal part of which the 

 optic nerve enters the eye. The inner walls of the choroidal fissure at 

 its hinder part may be regarded as continued into the optic stalk, and 

 it so happens that the optic nerve finally loses all connection with the 

 ou*^er wall of the optic cup and j)erforates it to connect itself with 

 the inner and thicker layer which has become the retina. The optic 

 stalk itself is, for the most part, if not entirely, converted into the optic 

 nerve and crus. With the further development of the optic cup, its rim 

 is reflected inwards and more fully covers the lens and becomes thinner. 

 After this stage it is said that in other forms the iris, with its muscles, 

 pigment, etc., are derived from the mesoblast around the choroid at the 

 rim of the optic cup. The cornea grows from a ring of cells of uncertain 

 origin just below the epithelial layer of the epiblast at the time the lens 

 is iuvagiuated, according to Balfour. The vitreous humor is developed 

 in much greater proportion at a very early stage in some forms than in 

 others. In Tylosurus and Apeltes it develops relatively early; in other 

 forms, as Gadus for example, it is not perceptible until about the time 

 of hatching. The vitreous humor appears to be gradually developed 

 and seems to me to be a fluid transudation ; perhaps it really arises as 

 an ingrowth through the choroidal fissure, as held by Balfour. The 

 aqueous humor develops, according to the last authority, within the ring- 

 like rudiment of the cornea, the cavity of which enlarges as development 

 advances. The brilliant silvery pigment of the iris of the young cod is 

 very probably of mesoblastic origin. Of the development of the mus- 

 cles of the eye little can be said here, as little is known of the subject. 

 The study of the development of the muscles, by the way, is a depart- 

 ment of embryology not yet sufficiently well cultivated. The blood- 

 vessels of the eye enter the organ through the choroid fissure. The eye 

 is already functionally active at a very early stage or upon the eve of 

 hatching, as I have detected movements of the eye-ball even just be- 

 fore the young fish had left the egg. Its complete pigmentation is ac- 

 complished by the time the embryo frees itself from the egg-membrane, 

 but most species of fishes do not begin to swim actively for some time 

 afterwards. In others of very rapid development, the movements which 

 are made are not sustained but fitful in character, and often executed 

 with astonishing velocity. That the eye is already functionally active 

 is proved by the ability to instantly recognize an approaching object 

 in the water manifested by young fishes a day or so did. They already 

 see well enough in most cases to try to avoid being caught with a pip- 

 ette or small skim net. 



The account w^hich precedes has dealt in large part with the evolu- 

 tion of the eye of the young fish and has carried us far beyond the time 

 when the neurula has been fully formed. Coincident with the increas- 



