AUDITORY OEGANS OF VERTEBEATA. 533 



processes of the upper jaWj and the external nasal processes by the 

 differentiation of these parts, and which leads from the region 

 of the inner angle of the eye towards the edge of the nasal pit, 

 sinks deeper down as these processes are developed (lachrymal 

 groove) ; it now becomes grown over by their edges so as to form a 

 canal, which, when the nasal cavity is developed, opens into it just 

 below the inferior turbinated bones. In the Eeptilia (Lacerta) it 

 opens near the posterior nares. At the inner angle of the eye 

 this lachrymal canal is divided into sevei'al smaller canals; there 

 are a larger number (3-8) of such canals on the lower eyelid of the 

 Crocodilini, but a smaller number (2) in Birds and Mammals. 



Auditory Organs. 



§ 400. 



The Auditory Organ of the Vertebrata, which has been ob- 

 served in all, except the Acrania, is derived from the ectoderm. 

 During the earliest embryonic period it is laid down in the form of 

 a thickening above the myelencephalon, which extends inwards. A 

 superficial organ of this kind, which must have carried the endings 

 of an integumentary nerve, must be regarded as the starting-point 

 of the great differentiation, which commences so early. The 

 earliest rudiment gives rise to a vesicle, which communicates with 



Fij?. 300. Development of the labyrinth of a Fowl. Vertical sections of the 



rudimentary skull, fl Pit of the labyrinth. Iv Vesicle of the labyrinth, c Kudimeut 



of the cochlea. Ir Eecessus labyrinthi. csp Posterior semicircular canal, cse Ex. 



ternal semicircular canal, jv Jugular vein (after Reissner). 



the exterior, and which is gradually cut off (Fig. 300), and enclosed 

 by the hinder lateral portion of the cartilaginous cranial capsule, 

 when that is differentiated. The primitive otocyst is the foundation 

 of a complicated cavitary system, in the walls of which the auditory 

 nerve is connected with its end-organs. From this is developed the 

 membranous labyrinth. The surrounding portions of the skull form 

 the cartilaginous, or osseous labyrinth. 



The simplest condition of the labyrinth is found in the Cyclo- 

 stomata. In the Myxinoidea a tracts which remains connected at two 



