488 HISTORY OF THE HUMAN BODY 



selachians and are retained with very little deviation by all 

 higher vertebrates. The sacculus, however, exists in a simple 

 form in fishes and shows considerable advance in the higher 

 forms. This advance consists of the gradual development of 

 a lateral sac, the lagena, which is situated upon the inner side 

 and which in fishes and amphibians is barely indicated. In 

 reptiles and birds the lagena becomes considerably elongated 

 and curved, and in mammals it becomes spirally wound and, 

 associating certain outside elements with itself, forms the 

 cochlea, here attaining the highest degree of complexity of 

 any part of the labyrinth. In the higher forms also the direct 

 connection between utriculus and sacculus becomes replaced 

 by an indirect one through the ductus endolymphaticus, which 

 arises by two branches, one from each of the two parts [Cf. 

 Fig. 134]. These unite and thus indirectly retain the con- 

 nection in question. 



The labyrinth of the cyclostomes stops at a lower point of 

 development than is represented by any of the gnathostomes 

 and may well represent the permanence of what is, in the 

 higher forms, an early embryonic stage. It consists of a simple 

 oval sac, not yet differentiated into utricnlus and sacculus, and 

 possessed of either one (Myxine) or two (Petromyzon) semi- 

 circular canals. Its endolymphatic duct, however, is short and 

 loses its connection with the exterior (Fig. 133, a). 



The walls of the embryonic labyrinth are composed of a 

 single layer of epithelial cells of appreciable thickness and all 

 alike; as development proceeds, however, the greater part of 

 the cells become flattened and form a transparent membrane, 

 while over certain definite areas, 6 to 8 in all, the cells are 

 thickened and columnar, forming neuro-epithelium. Certain 

 of these cells form the ultimate organs of hearing and are 

 provided with various sorts of terminal flagella and other 

 similar structures (auditory hairs}, which project into the lu- 

 men of the labyrinth and are bathed in the endolymph, i.e., the 

 fluid filling the interior. These are supplied with nerve fibers 

 from the auditory nerve. About these are placed various sorts 

 of supporting cells, which are without auditory function and 



