DEVELOPMENT OF FOURTH DAY 



515 



optic cup. The margins of the cup narrow, and converge toward the 

 lens, while the lens itself loses its connection with the superficial ecto- 

 derm and forms a completely closed vesicle. A microscopic study of 

 sections of the lens show an elongation of the cells on that side of the 

 lens which lies toward the center of the optic cup. These elongating 

 cells are to become the lens fibers. 



THE EAR 



The auditory placode has already been mentioned as forming on 

 the second day. This thickening of the ectoderm sinks below the sur- 

 rounding ectoderm and becomes the floor of the auditory pit. This sep- 

 arates from the superficial layer from which it formed. It will be re- 

 membered that this causes the auditory pit to lie close to the myelen- 

 cephalon. The tubular connection formed by the constriction of the 

 region between the sunken placode and the superficial layer where it 

 originally forms, remains open for a time as the endolymphatic duct. 



It is by a series of complicated changes that this placode, which 

 forms a vesicle, gives rise to the entire epithelial portion of the internal 

 ear mechanism. 



Nerve fibers from the acoustic ganglion grow inward to the brain 

 and outward to the internal ear, thus forming its nerve connections. The 

 external auditory meatus cannot yet be seen, nor has the dorsal and 

 inner part of the hyomandibular cleft as yet given rise to the Eustachian 

 tube, which is to form later. 



THE NOSE (Fig. 304) 



The olfactory pits are merely paired depressions in the ectoderm 

 of the head, ventral to the vesicles of the fore-brain and just anterior 

 to the mouth. These pits become deepened by the growth of the sur- 

 rounding parts. The epithelium of the pits ultimately comes to lie at 

 the extreme upper part of the nasal chambers, and there constitutes the 

 sensory epithelium. Nerve fibers grow inward from these cells to the 

 lobes of the fore-brain, constituting the olfactory or first cranial nerves. 



A 



Fig. 304. 



Olfactory region of the hen, A in transverse and B in longitudinal section. 

 c, middle concha; ch, choana ; i, inferior (anterior) concha; o, connection of air 

 cavity with head; p, septum of nose; s, superior concha. (From Kingsley after 

 Gegenbaur. ) 



