THE FORMATION OF THE ORGANS THE SENSORY ORGANS. 197 



tentacles of terrestrial Pulmonates, they are raised up as the tentacle 

 grows. The eyes are already visible when the tentacles are first 

 indicated (FoL). 



The next step in the development of the eyes is the deposition of 

 pigment in the proximal part of the optic vesicle. The cells of this 

 region increase and yield the retina, while the distal part forms the 

 cornea. Two structures arise within the optic vesicle as secretions 

 of the cells ; these are the lens and the vitreous body which are 

 at first homogeneous and strongly refractive. In the latter, delicate 

 fibres appear which run from the retina to the lens (v. ERLANGER, 

 N<>. 27). 



FIG. 89. A, Transverse section through a young embryo of Helix (Acavus) IValtoni, 

 /'>. one of the lateral organs (A, s) highly magnified (after P. and F. SARASIN). ect, 

 ectoderm ; mes, mesoderm ; s, lateral organs of the sensory plate ; si, posterior part 

 of the buccal mass, including transverse section of the oesophagus and the radular 

 s;n- : .s-n, sensory cells ; st, supporting cells. 



According to SALENSKY, the formation of the eyes, in Vermetus, is connected 

 with the invagination of the cerebral ganglion. They appear as rounded 

 thickenings at the edge of the plate which later sinks in to form the brain ; 

 these soon become hollow and form vesicles similar to those described above 

 as rudiments of the eyes, and then shift inwards in connection with the 

 iuvagination. Only somewhat late, when the lens has already been secreted, 

 do they lose their connection with the cerebral invagination. The further 

 development of the eyes apparently takes the course described above. This 



