n NERVOUS SYSTEM , 133- 



apparently is to detect slow vibrations in the water and is 

 therefore closely allied to hearing, it is of interest to notice that 

 the ectodermal rudiment from which they arise appears to be in 

 some cases continuous at first with that which gives rise to the 

 otocyst. 



The sense organs are in correlation with their origin at first 

 placed superficially but as development goes on they in most cases 

 become depressed beneath the surface either in isolated pits or in 

 continuous grooves. The latter may in turn remain open or may 

 become covered in to form tubes except where at intervals openings 

 remain leading to the exterior. This is the condition which is 

 reached in the adults of the majority of fishes. 



The lateral line sense organs being correlated with an aquatic 

 habit commonly degenerate on the assumption of a terrestrial 

 existence. Various Anura however which remain purely aquatic 

 after metamorphosis retain their full equipment of lateral line 

 organs. 



Organ of 1'inkus. — In Zepidosiren and Protopterus a peculiar 

 organ of special sense lies deeply embedded in the tissue on each 

 side of the head in close contact with the wall of the auditory 

 capsule. This organ, discovered in Protopterus by Pinkus, has been 

 shown by Agar (1906) to be developed from the ectodermal ingrowth 

 which forms the outer end of the spiracular rudiment. 



Eye. — As the eye develops in the same general manner, differing 

 only in detail, in the different subdivisions of the Vertebrata it will 

 be convenient to describe first its development in the Fowl — the 

 Vertebrate of which it is easiest to obtain material for practical 

 study. 



The first obvious rudiment of the eye consists of a projection of 

 the side wall of the thalamencephalon which juts out at right 

 angles to the axis of the body and gives a characteristic hammer- 

 shape to the fore-brain region (see Fig. 231, Chap. X.). A transverse 

 section across the head near its front end in a chick about the 

 middle of the second day of incubation shows (Fig. 234, D) 

 the thalamencephalon extending out on each side as the optic 

 outgrowth. 



As development goes on and mesenchyme accumulates between 

 the brain-wall and the ectoderm the proximal part of each optic 

 outgrowth becomes constricted, from above downwards, to form a 

 relatively narrow optic stalk (Fig. 74, A, B, D, o.s). The optic 

 outgrowth is closely apposed against the inner surface of the 

 external ectoderm and a slight thickening of the latter soon 

 becomes apparent just where it is in contact with the surface of 

 the optic outgrowth in (Fig. 74, B, I). This thickening is the 

 first rudiment of the lens. The lens-rudiment gradually becomes 

 sunk below the general surface to form a saucer- and later a 

 cup-shaped depression. As the rudiment becomes involuted in 

 this way, the outer wall of the optic outgrowth also becomes 



