No. 2.] DEVELOPMENT OF PETROMYZOIV. 297 



external to the recessns lahyrinthi. In the next stage (9 mm. 

 Fig. 31, PI. X) the two processes are in contact, and then 

 coalesce, dividing the vesicle by means of an oblique partition 

 into two chambers. In the middle of the dorsal wall the par- 

 tition is incomplete, and here the two chambers communicate, 

 as they do also at the anterior and posterior ends of the vesicle 

 The upper chamber is, of course, the rudiment of the semicir- 

 cular canals. Seen from above, the canals run obliquely from 

 the middle of the inner edge of the vesicle to its outer corners, 

 but with very slight curvature. In larvae of 13 mm. a median 

 invagination of the outer wall of the vesicle appears and forms 

 a prominence within the cavity, dividing the vestibule into the 

 two divisions already mentioned in the adult; in consequence 

 of this fold the semicircular canals take on a more pronounced 

 curvature (Fig. 32, PI. X). At the same time the appendix 

 is formed by a diverticulum from the vestibule ; at a later stage 

 its opening is narrowed by a constriction of the walls (Fig. 34, 

 PI. X). In larvs of 22 m. length all the essential parts of the 

 auditory organ are present, and later changes affect only the 

 form and proportions of the parts ; the capsule gradually in- 

 creases in height and breadth until it assumes the form seen in 



« 



the adult. 



The reccssiis lahyrinthi persists for a long time as a blind 

 membranous canal (Fig. 30-32, PI. X), the end of which lies 

 close to the brain and outside of the cartilaginous capsule of the 

 ear. It is plainly visible even in the larger larvae ; but I have 

 not been able to determine which structure, if any, of the adult 

 ear is derived from it. 



The early stages of the development of the auditory organ in 

 Petromyzon are altogether like those of the higher vertebrates, 

 but the ear remains permanently on a lower plane. No trace 

 of the horizontal semicircular canal occurs at any stage, and 

 from the time of the first appearance of the canals the structure 

 of the organ deviates widely from that found among other 

 vertebrates. It is a question as to whether these peculiarities 

 are to be regarded as primitive, or as the result of degeneration. 

 The much greater simplification of the labyrinth in Myxine 

 strongly suggests degeneracy ; but, on the other hand, the entire 

 absence of the horizontal semicircular canal may very well be 

 a primitive feature. It is plain that the auditory organ of the 



