DEVELOPMENT OF THE LENS 51 



The refractile fluid or substance may be first formed in the substance 

 of the cells, appearing as droplets in the cytoplasm ; thereafter it may be 

 retained in the substance of the cells, either in a semi-fluid condition or 

 a more solid state, as in the clear refractile rods or " rhabdites," or the 

 " crystal cones " of some types of simple or compound eyes ; or it may 

 be discharged as a secretion into the vitreous cavity. In some cases the 

 whole cell degenerates, the cell boundaries and nuclei disappear com- 

 pletely, and adjacent cells coalesce to form a viscous mass which afterwards 

 consolidates and assumes a lens-like form. In other cases two or more 

 cells may become enormously enlarged so that they fill the greater part 

 of the optic vesicle ; these become clear and highly refractile and may then 

 function as a lens or vitreous humour (Fig. 39). Considerable importance 

 has been attributed to the distinction between lenses which are cellular 

 and lenses which are non-cellular ; and also to the type of epithelium 

 which enters into their composition. The distinction is of importance 

 more especially with reference to the type of lens found in the lateral 

 eyes of vertebiates, which is cellular and epidermal in origin, and the 

 lens of the " pineal eye," which is also cellular but originates from the 

 distal wall of the optic vesicle, and is therefore derived from the ectoderm 



A — Cup-like depression of the body epithelium which forms the primary optic pit. 



B — Constriction of the mouth of the pit to form the primary optic vesicle. 



C — Formation of the iris fold and primary cornea. 



D — Development of the two halves of the non-cellular lens and the rampart-like 

 outer fold which gives rise to the secondary cornea. 



E — Further development of the secondary cornea, and formation of the anterior- 

 chamber. 



F — Section of the fully developed eye of Sepia officinalis, showing the retina, 

 optic ganglion, optic nerve, various parts of the cartilaginous capsule, muscle- 

 fibres, anterior chamber and the opening of the latter to the exterior : 



ant. ch. : anterior chamber. 



ap. : opening of anterior chamber to the exterior. 



b.e. : body epithelium, which becomes the outer epithelial layer (Corpus 



epitheliale externus). 

 cap. : cartilaginous capsule. 

 c. ep. : Corpus epitheliale. 



e.l. : part of lens formed by the outer epithelial layer. 

 i.e.l. : outer wall of optic vesicle, which becomes the inner epithelia 



layer (Corpus epitheliale internus). 

 i.f. : circular fold which becomes the iris. 

 id. : part of lens, formed by the inner epithelial layer. 

 mes. : mesenchyme. 

 m.f. : muscle fibres. 



o.f. : circular fold which forms the secondary cornea. 

 opt.g. : optic ganglion. 

 cp.n. : optic nerve. 

 pig. I. : pigment layer of retina. 

 rds. : rods. 

 r.l. : inner wall of the optic pit, which becomes the retina. 



(Redrawn from Lang's Comparative Anatomy.) 



