THE SENSORY ORGANS— THE EYES. 



299 



communicates with the exterior through an aperture. The sensory 

 epithelium is consequently directly bathed by the sea water, and its 

 ectodermal character thus becomes very evident. Eyes thus simply 

 (•(instituted have already been met with in a few primitive Gastropods 

 (Fig. 90, ]). 198). 



In the Dibranchiate Cephalopoda, the eye reaches a higher grade 

 ot development. The first advance is the closing of the primitive 

 optic vesicle and its abstraction from the ectoderm. Mesoderm-cells 

 then press in between the latter and the outer wall of the vesicle, a 

 process the commencement of which is indicated even in Fig. 112 H. 

 After the abstriction of the optic vesicle, this stage may he compared 

 to the permanent condition of the eye in the majority of the 

 Gastropoda ( Fig. 1 45 />). 



vd 



ff.VS 



Fig. 1 V-\. Transverse section through the bead of an advanced embryoof Loligo (aftei 

 Bobretzkt, from Balfour's Text-book), ac, otocyst ; adk, optic cartilage : ak and 

 //, lateral cartilage and white body : cc, iris ; //, funnel-fold : gc, cerebral ganglion ; 

 .'/"'. membrana limitans; gls, duct of the salivary gland : (ff.op), optic ganglion : (g.vs) . 

 visceral ganglion; rt, retina; vc, vena cava : vd, stomodaeum ; vk, ciliarj region 

 of the eye ; c, thickened ectoderm in the Boor of the tunnel. 



A second circular ectodermal fold now rises above the optic vesicle. 

 enclosing a depression which strongly resembles the primitive optic 

 pit (Fig. 1 l-~> B). Almost simultaneously, the (cuticular) secretion of 

 a conical structure (Fig. 143) commences on the inner surface of the 

 external wall of the vesicle; this is the first indication of the lens. 

 This rudiment increases in size through the deposit of concentric 

 layers (Fig. 1U A). 



