498 



THE AMMOCCETE EYE. 



Ammoccete 1 , in which it resembles in most important features that of other 

 Vertebrata. 



Eye of Ammocoetes. The optic vesicle arises as an outgrowth of 

 the fore-brain, but the secondary optic cup is remarkable in the young larva 

 for its small size (fig. 291, opv}. The thicker outer wall gives rise to the 

 retina, and the thinner inner wall to the choroid pigment. The lens is formed 

 as an invagination of the single-layered epidermis (fig. 291, /). As develop- 

 ment proceeds the parts of the eye gradually enlarge, and the mesoblast 

 around the hinder and dorsal part of the optic cup becomes pigmented. 

 There is at first no cavity for the vitreous humour, but eventually the 

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

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

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

 fissure of other types. The mesoblastic process 

 is probably the homologue of the processus 

 falciformis and pecten, and appears to give rise 

 to the vitreous humour ; for a long time it 

 retains its connection with the surrounding 

 mesoblast. Its cells eventually disappear, 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 optic cup forms a deep pit (fig. 292, r} ; 

 in the mouth of which is placed the lens (/). 

 The two walls of the retina have now the normal 

 vertebrate structure, though the pigment is as 

 yet imperfectly present in the choroid layer. 

 The lens has the embryonic forms of higher 

 types (cf. fig. 289), consisting of an inner thicker 

 segment, the true lens, and an outer layer form- 

 ing the epithelium of the lens capsule. The 

 edge of the optic cup, which forms the rudiment 

 of the epiblast of the iris, is imperfectly separated 

 from the remainder of the optic cup ; and a 

 mesoblastic element of the iris, distinct from 



-Oi c 



FIG. 291. HORIZONTAL 

 SECTION THROUGH THE 

 HEAD OF A JUST HATCHED 

 LARVA OF PETROMYZON 

 SHEWING THE DEVELOP- 

 MENT OF THE LENS OF THE 



EYE. 



th.c. thalamencephalon ; 

 op.v. optic vesicle ; /. lens of 

 eye ; h.c. head cavity. 



Descemet's membrane (di), 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 (i) the epidermis (cp] ; (2) the dermis 

 (dc) ; (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 appears to represent 

 Descemet's membrane. The subdermal connective tissue is continued as an 



1 The most detailed account is that of W. Miiller (No. 377). 



