No. I.] THE VERTEBRATE EAR. 211 



the two primary sense organs of the Cyclostome ear. The par- 

 ent sense organ soon divides transversely into two nearly equal 

 parts, which are the anterior and posterior sense organs respec- 

 tively. The auditory vesicle is at the same time partly sepa- 

 rated into two chambers (incomplete canals) to accommodate 

 them. These chambers are the utriculus and sacculus. 



Each of the two sense organs of the second generation after 

 a while divides into two unequal parts in such a manner that 

 the smaller sense organ appears as a bud from the parent. 

 There are thus formed within the two chambers of the ear four 

 canal sense organs belonging to the third generation. The two 

 external organs are soon inclosed within the ampulla of two 

 complete and relatively large canals, which are now formed 

 about them, — an anterior vertical and a posterior vertical, 

 formed in the manner already fully described. Up to this time 

 the organs have retained the primitive relations to each other — 

 a serial arrangement along a line running in an antero-posterior 

 direction. Now there begins a distortion of the structure as a 

 whole, which continues ever after, and reaches its greatest 

 development in the Mammals ; viz. a process of sinking and 

 drawing out ventrally of the posterior chamber of the ear, so 

 that hereafter we might speak of a superior chamber and an 

 inferior chamber. The sense organs of the third generation 

 play the leading role in these changes, and by means of four 

 sets of divisions, viz. by the bipartition of each sense organ 

 present in the Cyclostome stage nearly simultaneously, there is 

 produced a fourth generation of canal sense organs. 



To this fourth generation belong all the sense organs of the 

 internal ear of the higher vertebrates which in the diagram are 

 numbered 8 to 15 consecutively. 



During this process of sense-organ differentiation, the canals 

 have been variously modified, and to show the relationship of 

 the third generation to the fourth let us examine the diagrams 

 shown in PL IX, Figs. 2 and 3, in the construction of which I 

 have ignored the ventral distortion spoken of (Cut 25). 



The sense organ numbered 6 in the Cyclostome stage (the 

 crista acustica anterior, or anterior canal organ, of Myxine and 

 Petromyzon) has divided to produce sense organs 10 and 12 

 of the Gnathostome stage, i.e. the cristae acusticas anterioris 

 et horizontalis, of the human ear. 



