398 



PRIMARY OPTIC VESICLE. 



cerebral eyes. How far this is also the case with the compound eyes 

 of Insects is uncertain, in that it is quite possible that the latter eyes 

 may have had an independent origin. 



The relation between the paired and median eye of the Crustacea 

 is also uncertain. 



In the genus Euphausia amongst the Schizopods there is present a series 

 of eyes placed on the sides of some of the thoracic legs and on the sides of 

 the abdomen. The structure of these eyes, though not as yet satisfactorily 

 made out, would appear to be very different from that of other Arthropodan 

 visual organs. 



The Eye of the Vertebrata. In view of the various structures 

 which unite to form it, the eye is undoubtedly the most complicated 

 organ of the Vertebrata ; and though its mode of development is 

 fairly constant throughout the group, it will be convenient shortly 

 to describe what may be regarded as its typical development, and 

 then to proceed to a comparative view of the origin of its various 

 parts, and to enter into greater detail with reference to some of them. 

 At the end of the section there is an account of the accessory struc- 

 tures connected with the eye. 



The formation of the eye commences with the" appearance of a 

 pair of hollow outgrowths from the anterior cerebral vesicle or thala- 

 raencephalon, which arise in many instances, even before the closure 

 of the medullary canal. These outgrowths, known as the optic vesi- 

 cles, at first open freely into the cavity of the anterior cerebral 

 vesicle. From this they soon however become partially constricted, 

 and form vesicles (fig. 284 a), united to the base of the brain by 



comparatively narrow hollow stalks, 

 the rudiments of the optic nerves. 

 The constriction to which the stalk 

 or optic nerve is due takes place ob- 

 liquely downwards and backwards, so 

 that the optic nerves open into the 

 base of the front part of the thala- 

 mencephalon (fig. 284 h). 



After the establishment of the 

 optic nerves, there take place (1) the 

 formation of the lens, and (2) the 

 formation of the optic cup from the 

 walls of the primary optic vesicle. 



The external or superficial epiblast 

 which covers, and is in most forms in 

 immediate contact with, the most pro- 

 jecting portion of the optic vesicle, 

 becomes thickened. This thickened 

 portion is then driven inwards in the form of a shallow open pit 

 with thick walls (fig. 285 A, o), carrying before it the front wall (r) 

 of the optic vesicle. To such an extent does this involution of the 



Fig. 284. Section through the 



HEAD OF AN EMBRYO TeLEOSTEAN, TO 

 SHEW THE FORMATION OF THE OPTIC 



VESICLES, ETC. (From Gegenbaur; 

 after Schenk.) 



c, fore-brain ; a. optic vesicle ; b. 

 stalk of optic vesicle ; d. epidermis. 



