528 



COMPARATIVE ANATOMY. 



I e 



walls of the primitive optic vesicle close to one another ; the whole 



then forms the se- 

 condary optic ves- 

 icle, and is cup- 

 shaped; the month 

 of the cup is 

 filled by the ec- 

 todermal thicken- 

 ing. This latter 

 forms the rudiment 

 of the lens (I). 

 While the stalk of 

 the primary vesicle 

 is being converted 

 into the optic nerve, 

 the tissue behind, 

 which is enclosed 

 by these parts, is 

 converted into 



Fig. 291. A Vertical section through the rudimentary 

 head of a Fish. c Brain, a Primitive optic vesicle. 

 b Its stalk, d Integumentary layer. B Formation of the 

 secondary optic vesicle, p Outer, r Inner layer of the 

 primitive optic vesicle, e Epidermis pushing the lens (I) 

 into the secondary optic vesicle. The vitreous body is seen 

 behind (after S. Schenk). 



t 



substance, which gradually fills up the greater part of the secondary 

 optic vesicle, and forms the vitreous body. The innermost layer of 

 tissue around the secondary optic vesicle is converted into a vas- 

 cular membrane, the choroid, while a firm fibrous layer outside it 

 forms the sclerotic, and invests the secondary optic vesicle ; this 

 grows out anteriorly as far as the connection between the lens and 



the ectoderm. As a result of the extension of 

 this process, the lens becomes cut off, and a trans- 

 parent portion of the sclerotic now intercalated 

 in front of it forms the cornea, which at the 

 same time becomes connected with the rudi- 

 mentary piece of integument (conjunctiva) 

 which lays in front of the lens. 



The eye then is a rounded capsule (bulbus 

 oculi), the investment of which (sclerotic) ex- 

 tends over the optic nerve, and is thence con- 

 tinued into the dura mater, while anteriorly 

 it is continued into the cornea. Within this 

 capsule is the secondary optic vesicle, which 

 is developed from the invaginated primary 

 one, and which is separated from the sclerotic 

 by the choroid. The secondary vesicle, in 

 which there is a lateral cleft owing to the 

 ingrowth of the "vitreous body," embraces the lens anteriorly. 

 These two, layers (a b), which pass into one another at this 

 anterior margin, and at the lateral fissure (Fig. 295, s), are not 

 similarly differentiated ; the inner one (l>), which is greatly thick- 

 ened at a very early stage, has its hinder portion converted into 

 the retina, while the outer, and thin one {a), forms the tapetum 

 nigrum. When the tapetum nigrum becomes pigmented, a pale 



Fig. 295. Section 



through the secondary 

 optic vesicle of a Fish's 

 embryo, taken vertically 

 to the "choroidal fis- 

 sure s." a Outer, b Inner 

 lamella of the optic 

 vesicle, c Vitreous body. 

 d Lens (after S. Schenk). 



