Ear and Nose 



419 



two posterior halves. Partial twinning, in- 

 volving either the semicircular canals, which 

 develop from the ventral half of the ear 

 plate, or the saccule, which develops from 

 the dorsal half, also occvurs" (Harrison, '45, 

 pp. 295-296). 



On the basis of his experiments, Harrison 

 has concluded that the ectoderm of the ear 



of harmonic equipotentiality after fixation 

 of the AP axis and before that of the DV 

 axis. These observations led to further in- 

 vestigations on the polarity of the ear ecto- 

 derm. 



In the experiments indicated above, both 

 host and donor are at the same stage of 

 development for any one experiment. As has 



cr.l 



p.a 



m.s 

 TWO ANTERIOR HALVES 



TWO POSTERIOR HALVES 



Fig. 149. Normal right labyrinth and two types of enantiomorphic twins, medial surface. The large arrow 

 points to the posterior end of the embryo; the small arrows point posteriorly with respect to the labyrinth or 

 half labyrinth to which they refer, c.a. Anterior semicircular canal; c.p, posterior canal; cr.a and cr.p, 

 the corresponding sensory cristae; cr.l, lateral crista, projected from the ventrolateral wall but not 

 visible in this view; d.e, endolymphatic duct; m.l, macula of lagena; m..s, macula of sacculus; m.u, macula 

 of utriculus; p.a, papilla amphibiorimi; s.e, endolymphatic sac (Harrison, '45). 



region and the surrounding area is at first 

 isotropic about the axis perpendicular to its 

 surface. Just as the neural folds are closing 

 a change supervenes. The ear plate becomes 

 polarized with respect to its AP axis. A little 

 later conditions again change, indicating a 

 transition from an indifferent DV axis to 

 one which is definitely polarized. A correla- 

 tion between the polarization of the ear 

 ectoderm and a regularity in the molecular 

 arrangement of the constituent cells has 

 been postulated and an attempt to demon- 

 strate this by x-ray diffraction, though un- 

 successful, indicates a possible means of 

 extending fundamental knowledge of em- 

 bryology (Harrison, Astbury, and Rvxdall, 

 '40). 



In Amblystoma Hall ('37) studied laby- 

 rinths whose ectoderm had been rotated 90 

 degrees. In these experiments, the ear rudi- 

 ment may still respond to some of the tests 



been indicated, conditions in the hosts as 

 well as the grafts are variables. When the 

 stage of either host or donor is constant in 

 a series, a further analysis of the problem 

 is possible; this has been done in hetero- 

 stadic transplants to homeotopic and hetero- 

 topic positions in Amblystoma (Hall, '39, 

 '41; Yntema, '39). The results of these ex- 

 periments call for modifications in the con- 

 clusions drawn from homostadic operations 

 (Fig. 148B-£). Expression of polarity de- 

 pends in part on the strength of polarization 

 in the rudiment, in part on the intensity of 

 the polarizing factors in the environment. A 

 labile AP polarity is found in the ear rudi- 

 ment of the early neural plate stage; this 

 anisotropism is masked in Harrison's experi- 

 ments by the ability of the hosts used to 

 reverse it. A similar inclination along the 

 DV axis was not demonstrated. Host factors 

 concerned with the arrangement of struc- 



