VISUAL OEGANS OF VERTEBRATA. 



527 



will be bestj consequently, to consider tliis new arrangement of the 



nasal cavity, when we come to treat of 



the mouth. Glands are differentiated 



in the mucous membrane of the nasal 



cavity, which are of a relatively large 



size in the Amphibia, although not absent 



in the Mammalia. Connected with the 



process by which the primitive nasal pit 



is carried far inwards, is the development 



of an organ, which appears to be a part 



separated off from the nasal pit. This is 



the organ of Jacobson. It forms a 



tube which lies at the base of the nasal 



cavity, and ends blindly behind (Fig. 293, 



J) ; the olfactory fibres in its walls are 



provided with end-organs. These organs 



are found in Keptiles and Mammals, and Fig. 293. Section tlurough the 



. , ,-, 1 '^ 1 •. 1 11 J i. nasal cavity and Jacobson s 



open mto the buccal cavity by the ducts oj.gan. sn Septum (after J. v 



of Stenson. Lacerta). 



Visual Organs. 



§ 397. 



The eye in the Vertebrata appears to have essentially the same 

 structure as in the more highly developed groups of lower 

 animals ; but the ontogeny of the organ shows that it belongs to 

 another type, and this is also obvious from its minute structure. 

 We cannot, therefore, connect it directly with the relatively well- 

 developed stages of the eye in other animal phyla ; the only indi- 

 cations of any connection are to be seen in the Tunicata. In the 

 larv» of the Ascidiae, as in Vertebrata, the eye is not directly de- 

 veloped from the ectoderm, but from the anterior portion of the central 

 nervous system. What is known as the eye in Amphioxus is of a 

 much lower grade ; it is a spot of pigment which varies in character, 

 and is attached to the anterior end of the central nervous system. 



The central nervous system chiefly, and secondarily the integu- 

 ment, are concerned in the constitution of the Vertebrate eye. The 

 former gives rise to the apparatus which perceives, the latter to the 

 apparatus which refracts, the light. The earliest rudiment of the eye 

 is a diverticulum, which is developed from the sides of the prosen- 

 cephalon (Fig. 294, A a), and which has the form of a vesicle, connected 

 by a stalk (h) with the rudiments of the brain (c). The "primitive 

 optic vesicle " lies below the ectoderm ; the ectoderm next gives rise 

 to a thickening {B), which pushes in the anterior wall of the vesicle 

 towards the posterior one. Below this thickening a process of the 

 mesoderm grows towards the optic vesicle, and puts the side-walls 

 also of that vesicle in continuity with the epidermic thickening. 

 The effect of these processes is to bring the anterior and posterior 



