II 



OTOCYST 



131 



not appear synchronously — the anterior vertical appearing first, then 

 the horizontal and finally the posterior vertical. The same order is 

 followed in subsequent development the anterior vertical canal keep- 

 ing ahead of the other two — probably a foreshadowing of the greater 

 importance of this canal, parallel to the sagittal plane, in the function 

 of flight. 



It should be noticed that the posterior vertical canal assumes its 





Fio. 73. 



-Development of the otocyst in Lung-fishes as seen in transverse sections. 

 (Prom drawings by M. C. Cairney.) 



A, Jstpidosiren, stage 21 ; B, Protopterus, stage 23 ; C, Lepidosiren, stage 28. N, notochord ; ot, 

 otocyst ; rh, rhombencephalon. In Fig. C the rudiment of the endolymphatic duct is visible as an out- 

 growth from the otocyst wall dorsally and mesially. 



position at right angles to the planes of the other two secondarily. 

 At first its rudiment is continuous with and almost in the same plane 

 as the anterior canal but as it assumes the tubular form it swings 

 outwards and forwards slipping, as it does so, over the horizontal 

 canal in the way indicated in Fig. 72, F, G, H. 



The otocyst of Vertebrates in general develops along similar lines 

 to those described for the Fowl while presenting modifications in 

 detail. The sensory epithelium is a development of the deep layer 

 of the ectoderm and in cases where the ectoderm is distinctly two 



