EMBRYOLOGY OF TADPOLE AND CHICK 581 



In the chick, and in fact in all vertebrates except the Teleosts, the 

 lens forms as a hollow vesicle caused by the surface ectoderm invagi- 

 nating. The choroid fissure of the optic cup closes a day or two before 

 hatching. The closing begins opposite the pupil. 



THE EAR 



The auditory placode appears just as the neural folds close. These 

 placodes then become depressed below the surface of the head and invagi- 

 nate to form the auditory sac or otocyst. 



This sac at the time of hatching has become completely closed and 

 separated from the ectoderm from which it arose. Therefore, it comes 

 to lie in close relation to the lateral surface of the medulla. 



The superficial layer of ectoderm continues to remain as a covering 

 of the region where the placode invaginated. The wall of the auditory 

 sac is but one cell in thickness, except in the medio-ventral region. It 

 is in this region that the ganglionic part of the placode is located. There 

 is a small finger-like outgrowth from the sac, which extends dorsally 

 from the medio-dorsal region. This is to become the endolymphatic 

 duct. 



There is little change in the ear region from this time to the opening 

 of the mouth, that is, until the tadpole is ten to twelve millimeters in 

 length. Then development seems to begin again. 



The remaining complicated changes in the formation of the inner 

 divisions of the ear are beyond the scope of this book. The VIII cranial 

 nerve connects with the ear. 



THE NOSE 



The olfactory organs appear quite early, in fact before the brain 

 closes (Fig. 339, no). A pair of ectodermal thickenings appears on each 

 side of the head just above and anterior to the future mouth region. 

 Again, only the deeper nervous layer is involved, as with the formation 

 of the ear; only in this case, the superficial layer of ectoderm does not 

 remain as a covering but disappears entirely. The thickenings them- 

 selves lie immediately anterior to the lens placodes, and are called olfac- 

 tory placodes. 



The placodes invaginate, forming the olfactory pits which are later 

 to become the true nasal cavities (Fig. 339). A few cells from the inner 

 surface of the olfactory placode become detached and come into com- 

 munication with the surface of the fore-brain to form a sort of crest- 



munication of the lateral and lower region of the olfactory cavity ; md.h., wall of 

 the mouth-cavity ; no,, external nares ; o, upper region of the olfactory cavity ; 

 olf, olfactory nerve ; o-u, communication between the upper and lower regions of 

 the nasal cavities ; u, lower region of the nasal cavity. ( After Bancroft. ) 



D, Frontal section of human fetus of 29 mm. (After Tourneux.) 



E, Sections through the nasal region. Bone black and cartilage dotted, ds, den- 

 shelf ; g, Jacobson's glands; j, Jacobson's organ; n. main cavity of nose; oj, 

 opening of Jacobson's organ; t, tooth-germ. (After Schimkewitsch.) 



