ii OTOCYST 129 



A curious, possibly adaptive, arrangement has been noticed in 

 late developmental stages of certain Sauropsida, where for a time the 

 external nares are plugged by a proliferation of ectoderm (Apteryx — • 

 T. J. Parker, Sphenodon — Dendy). Such temporary obliteration of 

 a channel at a period of development where it is unnecessary or 

 harmful is a phenomenon which occurs fairly frequently : examples 

 of it will be met with later in connexion with the alimentary canal 

 and the excretory organs. 



Otocyst. — The Vertebrate possesses a pair of otocysts situated 

 one on each side of the hind-brain. Each arises in the typical 

 fashion familiar in the invertebrates, by a sensory portion of ecto- 

 derm becoming depressed below the surface of the skin and eventu- 

 ally isolated as a closed vesicle. As in the invertebrates certain 

 of the lining cells of the otocyst secrete otolithic masses of Calcium 

 carbonate. 



The otocyst of the Vertebrate however shows two developments 

 which do not occur amongst the invertebrates. Firstly, in connexion 

 with the primitive function of the organ, that of balancing, the wall 

 of the growing otocyst becomes moulded into the three semi- 

 circular canals which are arranged in planes at right angles to one 

 another. These canals have for their function the analysis of any 

 rotatory movement into its components in these three planes. And 

 secondly a special region of the otocyst wall becomes specialized in 

 connexion with a new sense — that of hearing — and grows out into 

 a curved horn-like pocket, the lagena, which may become greatly 

 enlarged and spirally coiled, in Vertebrates in which the sense of 

 hearing is very acute, forming the organ known as the cochlea. 



The development of the otocyst may be described as it occurs in 

 the Fowl (Rothig and Brugsch, -1902). The otocyst begins to make 

 its appearance during the second day of incubation as a thickened 

 area of ectoderm on each side of the hind-brain. This thickened 

 area becomes depressed below the general surface, forming a saucer- 

 shaped depression which gradually deepens till it forms a deep pit. 

 The lips of this pit, especially dorsally, grow inwards so as to constrict 

 the opening which is finally completely obliterated, the original open 

 depression being thus converted into a closed, somewhat pear-shaped, 

 sac the otocyst. The wall of the otocyst remains for a time con- 

 tinuous through a solid bridge with the outer ectoderm (Fig. 72, A) 

 but as a rule this bridge persists merely for a very short time and 

 only a small cellular tag attached to the otocyst remains to mark its 

 position. 



As development goes on the otocyst increases in size by growth 

 of its wall and this growth is especially marked ventrally and later- 

 ally with the result that the point which was originally connected to 

 the ectoderm- becomes displaced so as to be situated on the mesial 

 side of the otocyst. This portion of the otocyst wall now comes to 

 project upwards as a distinct pocket-like outgrowth the recess 

 (Fig. 72, B, r). External to this a wider bulging of the wall fore- 



VOL. II K 



