410 



THE AMMOCAETE EYE. 



growth of the optic cup gives rise to a space, into which a cellular process 

 of Diesoblast grows at a slight notch in the ventral edge of the optic cup 

 (W. Miiller, No. 377). This notch is the only rudiment of the choroid 



fissure of other types. The mesoblastic 

 process is probably the honiologue of the 

 processus felciformis and pecten, and ap- 

 pears to give rise to the vitreous humour; 

 for a long time it retains its connection 

 with the surrounding mesoblast. Its cells 

 eventually disa))pear, and it never contains 

 any vascular structures. 



The lens for a long time remains as 

 an oval vesicle with a central cavity. In 

 a later stage, when the Ammoccete is fully 

 developed, the secondary o})tic cup forms 

 a deep pit (fig. 292 r) ; in the mouth of 

 which is placed the lens {I). The two 

 walls of the retina have now the normal 

 vertebrate structure, though the pigment 

 is as yet imperfectly present in the cho- 

 roid layer. The lens has the embryonic 

 forms of higher types (cf fig. 289), con- 

 sisting of an inner thicker segment, the 

 true lens, and an outer layer forming the 

 epithelium of the lens capsule. The edge 

 of the optic cup, which forms the rudi- 

 ment of the epiblast of the iris, is imper- 

 fectly separated from the remainder of 

 the optic cup ; and a mesoblastic element of the iris, distinct from Desce- 

 met's membrane (tZm), can hardly be spoken of 



There is no cavity for the aqueous humour in front of the lens ; and 

 there is no cornea as distinct from the epidermis and subepidermic tissues. 

 The elements in front of the lens are (1) the epidermis (ep) ; (2) the dermis 

 {(Ic) ; (3) the subdermal connective tissue {sdc) which passes without any 

 sharp line of demarcation into the dermis; (4) a thick membrane, con- 

 tinuous with the mesoblastic part of the choroid, which a])pears to represent 

 Uescemet's membrane. The subdermal connective tissue is continued as 

 an investment round the whole eye ; and there is no difierentiated sclerotic 

 and only an imperfect choroid. 



In a still later stage a distinct mesoblastic element for the iris is 

 formed. When the Ammoccete is becoming a Lamprey, the eye approaches 

 the surface ; an anterior chamber is established ; and the eye differs 

 from that of the higher types mainly in the fact that the cornea is hardly 

 distinguished from the remainder of the skin, and that a sclerotic is very 

 imperfectly represented. 



Optic vesicles. The development of the primitive optic vesicles, so 

 far as is known, is very constant throughout the Vertebrata. In Teleostei 

 and Lepidosteus alone is there an important deviation from the ordinary 

 type, dependent however upon the mode of formation of the medullary 

 keel, the optic vesicles arising while the medullary keel is still solid, and 

 being at fii-st also solid. They subsequently acquire a lumen and undergo 

 the ordinary changes. 



Fio. 291. Horizontal section 



THROUGH THE HEAD OF A JUST 

 HATCHED LARVA OF PeTROMYZON 

 SHEWING THE DEVELOPMENT OF THE 

 LENS OF THE EYE. 



th.c. thalamencephalon ; qj>.v. 

 optic vesicle; I. lens of eye; h.c. 

 head cavity. 



