Ear and Nose 



417 



tion rather than an inductor of the ear along 

 with the facial ganglion as held by Szepsen- 

 wol ('33). 



Some of the results indicated above in- 

 volve comparisons between a system at one 

 stage with the same system at another stage. 

 As will be evident in later discussion, the 

 states of one variable in such a continuum 

 are not obvious unless other variables are 



the four cardinal positions so that in respect 

 to the two main axes of the ear, both, 

 either one, or none are harmonic with the 

 host axes (Fig. 147). The resulting vesicles 

 may be classed as belonging to four types: 

 (1) harmonic, (2) disharmonic, (3) redupli- 

 cated, (4) irregular or vesicular. In trans- 

 plants with the AP axis disharmonic, the 

 asymmetry is reversed and a harmonic 



V V V V 



Fig. 147. Scheme of operations to test the effect of changed orientation. The profile of an Amblystoma 

 embryo (stage 21) is shown above. The curved broken line represents the embryonic axis; the stippled area, 

 the rudiment of the ear placode; the square area, the graft. The four squares below show the positions of 

 the axes in four orientations for the right side of the embryo; R, a graft from the right side; L, a graft from 

 the left side. The letters inside the squares give the orientation of the graft; those outside, the direction of 

 the cardinal poiats of the embryo (Harrison, '45). 



held constant. The results do show a pro- 

 gressive loss of ear-forming ability on the 

 part of the system as a whole but do not 

 demonstrate whether such loss is due to 

 changes in one or more parts of the system. 



POLARIZATION OF THE EAR 

 ECTODERM 



Before and during development of a mo- 

 saic pattern the ear rudiment of Amblystoma 

 becomes irreversibly polarized along its main 

 axes, anteroposterior (AP) and dorsoventral 

 (DV), as the experiments of Harrison ('24, 

 '36a, '36b, '45) have shown. The ear rudi- 

 ment from a donor is grafted into the ear 

 region of a host at the same stage of de- 

 velopment as the donor (homostadic trans- 

 plantation). The graft is placed in one of 



ear develops in a majority of experiments 

 done when the neural folds are approxi- 

 mated but not fused (Fig. 148/4, stage 19). 

 In later stages the disharmonic AP polarity 

 is usually not reversed. The DV axis be- 

 comes fixed during tail-bud stages (stages 

 25-27); later rotations of the DV axis tend 

 to produce disharmonic labyrinths. Simi- 

 larly, the AP axis is reversible in Bufo 

 when the neural folds appear but is fixed 

 when the neural folds are fused (Choi, '31). 

 If the AP axis is reversed during critical 

 stages at the end of neurulation in Am- 

 blystoma the polarization of its ectoderm 

 is frequently adjusted only partially to its 

 new environment and an enantiomorphic 

 twin ear results (Fig. 149). "These are al- 

 ways mirrored across the transverse plane 

 and may consist of either two anterior or 



