CYCLOSTOMES 



261 



THE AMMOCCETE EYE 



Since the original description by W. Miiller (1875), several studies 

 have been made of the animocoete eye. The youngest specimen 

 described was that of Ida Mann (1928) who figured a simple optic 

 vesicle evaginated from the anterior cerebral vesicle lying close 

 underneath the surface ectoderm (Fig. 271). At this early stage 

 there was neither vitreous nor lens, the outer layer of the vesicle 



mM^m 



Fig. 271. — Section Through the Eye of the Ammoccetes (the Larva 



OF PETROMYZOy FLUVIATlLIs). 



There is neither vitreous nor lens ; the optic cup is closely folded upon 

 itself, the outer layer being pigmented and the inner showing a considerable 

 degree of differentiation. 



a, surface epithelium of the head ; 6, pigmented outer layer of the optic 

 cup ; c, nuclei of the visual cells ; d, nuclei of bipolar cells ; e, ganglion cells 

 with nerve fibres arising from them ; /, visual cells ; g, muscle mass of head ; 

 h, optic nerve (Ida Mann). 



being pigmented and the inner showing differentiation into the three 

 layers of cells characteristic of the vertebrate visual retina — visual 

 cells (indistinguishable either as rods or cones), bipolar cells and gan- 

 glion cells the axons of which constitute the oj^tic nerve. In somewhat 

 older larvae (5-10 mm.), von Kupffer (1894) and Studnicka (1912) des- 

 cribed a lens vesicle lying underneath the single layered ectoderm 

 and completely separate from the ojjtic vesicle (Figs. 272 and 273), 

 while Carriere (1885) in a more mature larva (30 mm.) described 

 a lens, at this stage still vesicular, invagiTiated within the optic 

 vesicle. Eventually the lens becomes solid, tlie anterior and vitreous 



