573 SCIENCE PROGRESS 



of the body — the sense organs in the canals being probably 

 stimulated by the impingement against them of the fluid in the 

 canals displaced by rotational movements of the head, the 

 otolith organs being, in a similar way, stimulated by the drag or 

 pressure of the otoliths upon them during movements in direct 

 lines or through alteration in the resting position of the body. 1 



In land vertebrates, as we shall see later, an additional 

 sense organ arises between the sacculus and its lagenar appen- 

 dage (fig. 2, Mac. bas.) and undergoes progressive elaboration 

 to form in conjunction with parts of the perilymph system a 

 special auditory organ independent of the vestibular parts of 

 the labyrinth. 



Such an organ as the above typical endolymph labyrinth, 

 even before the advent of the cochlea, is obviously very far 

 removed from a simple lateral-line sense organ but the relation- 

 ship between the two can be traced in the early development 

 of the ear. 



At its first appearance 2 the ear, like many other epidermal 

 sense organs, is a little superficial thickening. Further growth 

 transforms this into a pit, which sinks deeper and deeper into 

 the mesodermal tissues, becoming a long-necked flask, like one 

 of the isolated lateral-line organs of the skin, with a single area 

 of modified epithelium to represent the sense organ. This 

 may be considered to represent the lateral-line stage of the ear. 



In most cases the flask now becomes nipped off from the 

 surface and begins to develop characteristics peculiar to the ear. 



The first indication of anything distinctive is the formation 

 of a narrow fold along the upper border of the vesicle. 3 This 

 is the budding canal system and heralds the formation of the 

 two canals that lie in the vertical planes. The horizontal canal 

 in almost all cases appears later. This sequence in the canal 

 formation is particularly interesting, for in the few cases (Hag- 

 fishes and Lampreys) where there are only two canals, it is the 

 horizontal canal that is missing. 4 



Variation in the form and relative length and width of the 

 semi-circular canals is decidedly capricious r ° ; frequently a canal 



1 Lee, Jour. Physiol. 15, 1894, p. 311, and 17, 1894-5, p. 192. 



2 Krause, Handbuch der Entwicklungslelwe, L. 4 and 5, 1893. 



3 Krause, Arch. Mikr. Anat. Bd. 35, 1890, p. 287 ; Fleissig, Anat. Hfte. 37, 

 1908, p. 69. 



4 Tretjakoff, Anat. Anz. 32, 1908, p. 165. 



5 Wulf, Arch.f. Anat. 1901, p. 57. 



