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 

 I'ctodermal character thus becomes very evident. Eyes thus simply 

 constituted have already been met with in a few primitive Gastropods 

 (Fig. 90, p. 198). 



In the Dibranchiate Cephalopoda, the eye reaches a higher grade 

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

 optic vesicle and its abstriction 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. 142 B, 

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

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

 Gastropoda (Fig. 145 B). 



x vc 



Fi<;. 143. Transverse section through the head of an advanced embryo of Loligo (after 

 BOBRETZKY, from BALFODR'S Text- book), ac, otocyst ; adk, optic cartilage ; a&and 

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

 <jin, membranalimitans; gls, duct of the salivary gland ; (g.<>p), optic ganglion ; (g.vs) , 

 visceral ganglion ; rt, retina ; vc, vena cava ; vd, stornodaeum ; vk, ciliary region 

 of the eye ; ./, thickened ectoderm in the floor of the funnel. 



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

 enclosing a depression which strongly resembles the primitive optic 

 pit (Fig. 145 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. 144 A}. 



