THE EMBRYOLOGY OF THE OPOSSUM 193 



that in the opossum the eyelids have become fused together 

 and covered over by the epitricium. Externally there is no 

 indication of the eye, except that in the living specimens the 

 pigment of the retina can be faintly seen through the epi- 

 tricium. 



I have removed the lens from a newborn opossum, and found 

 that 15 days later (17183) there was no regeneration, and in 

 the absence of the lens the cornea had not cleared and the 

 retina had atrophied to a considerable extent. The control 

 eye in the same individual had advanced to a stage similar to 

 that of a 3-month human foetus. 



Like the eye, the ear at birth is completely non-functional 

 (Lars-ell, McCrady and Zimmermann, '35). The cochlear 

 duct (fig. 56) has grown out to about one-half turn. It con- 

 tains no organ of Corti, but the epithelium of the tympanic 

 wall is slightly thicker than that of the vestibular wall. There 

 are no hair cells, tunnel of Corti, tectorial membrane, scala 

 tympani, or scala vestibuli. The external auditory meatus 

 is completely plugged with epitrichial cells, and the latter 

 also overlie the developing pinna. 



In the vestibular portion of the inner ear the ampullae of 

 the semicircular canals are recognizably indicated, and the 

 utricular and saccular anlagen, though not yet separated from 

 each other, are identifiable. The only indication of the cristae 

 and maculae is the slightly thicker epithelium at the appropri- 

 ate points. There are still no signs of cupulae, otolithic mem- 

 branes, or perilymphatic space. No nerve endings are 

 present in either the acoustic or vestibular part of the inner 

 ear. 



The olfactory organs, on the other hand, may possibly be 

 in a functional condition. The conchae are not well developed 

 (about like those in a 3-month human foetus), but the olfac- 

 tory epithelium is ciliated, and nerve fibers from this epi- 

 thelium enter the olfactory lobe of the forebrain in large 

 numbers. Selenka, also judging from histological appear- 

 ances, considered this organ functional. 



There are no papillae or taste buds in the tongue. 



