492 



EMBRYOLOGY. 



body in the form of a long narrow tube, which traverses the cartila- 

 ginous primordial cranium and is in union dorsally with the epidermis 

 at the surface of the body, where it possesses an external opening. 



In its first, fundament the organ of hearing in Vertebrates resembles 

 in the highest degree those structures which in the Invertebrates are 

 interpreted as organs of hearing. These are lymph-filled vesicles lying 

 under the skin, which are likewise developed out of the epidermis. 

 Either they are wholly constricted off from the epidermis, or 

 they remain connected with it by means of a long, ciliate, epithelial 

 canal, as in the Cephalopods, even after they have become surrounded 



by connective tissue. In both 

 n h cases the vesicles are lined 

 with epithelium which con- 

 sists of two kinds of cell : 

 first of low, flat elements, 

 which ordinarily exhibit ciliary 

 movements and thereby put 

 in motion the fluid within the 

 vesicle, and secondly of longer 

 cylindrical, or thread-like, au- 

 ditory cells with stiff hairs, 

 which project into the endo- 

 lymph. The auditory cells are 

 either distributed individually 

 over the inner surface of the 

 auditory vesicle or arranged 

 in groups, or they are united 

 at a particular place into an 

 auditory epithelium, the au- 

 ditory patch (macula acustica) 

 or the auditory ridge (crista acustica), which may be either single 

 or double. To all the auditory vesicles of the Invertebrates there 

 is sent, moreover, a nerve which ends at the sensory cells in fine 

 fibrillae. Finally, there is present as a characteristic structure a 

 firm, crystalline body, the otolith, which is suspended in the midst 

 of the endolymph and is ordinarily set in vibration by the motion 

 of the cilia. It consists of crystals of phosphate or carbonate of 

 lime. 



Sometimes there is only a single large, in most cases concentrically 

 laminated, spherical body, sometimes a number of small calcareous 

 crystals, which are held together by means of a soft pulpy substance. 



Pig. 275. Vertical [cross] section through the 

 vesicle of the labyrinth of an embryo Sheep 

 1'3 cm. long, after BOETTCHER. Magnified 30 

 diameters. 



nh, Wall of the after-brain ; rl, recessus labyrinth! ; 

 Ib, vesicle of the labyrinth ; yc, ganglion coch- 

 leare, which is in contact with a part of the 

 labyrinth-vesicle (dc) that grows out into the 

 ductus cochlearis. 



