Section VII 



CHAPTER 3 



Ear and Nose 



C. L. YNTEMA 



LOCALIZATION OF RUDIMENTS 



The positions of the presumptive areas of 

 the auditory and nasal placodes as indicated 

 in the diagrams have been determined pri- 

 marily by vital staining in Amphibia and 

 by explantations in the chick (Figs. 145 

 and 146). In the former group the placodal 

 areas are underlaid by chordamesoderm dur- 

 ing late gastrular stages. The ear ectoderm 

 comes to lie next to the myelencephalic part 

 of the neural tube; the paired areas of nasal 

 ectoderm are closely associated for a time 

 with the neural folds of the prosencephalon. 



THE EAR 



DIFFERENTIATION OF EAR ECTODERM 

 A differentiation of the ear rudiment from 

 adjacent cells is present in the late gastrula 

 of Amblystoma (Yntema, '39) and in the 

 early neurula of Rana (Zwilling, '41). Obvi- 

 ously, this demonstrated determination does 

 not imply either presence or lack of an 

 earlier differentiation. Since this differentia- 

 tion in late gastrular and early neurular 

 stages can be reversed easily, its demonstra- 

 tion depends upon using an environment 

 favorable to expression of the tendency to 

 form an ear vesicle. Transplantation to the 

 region between the eye and ear rudiments 

 of a head process stage serves this purpose 

 in urodeles. Adjacent structures influence the 

 transplanted ectoderm, but only ear ecto- 

 derm gives rise to ear vesicles in this position. 

 During neurulation differentiation of the 

 rudiment advances; increasingly normal ears 

 form from ectoderm transplanted to favor- 

 able sites and determination is acquired to 

 develop in less favorable environments. This 

 has been demonstrated or is implied by the 

 results of several investigators on amphibian 

 embryos (Kaan, '26; Rohlich, '29; Yntema, 

 '33; Domacavalli, '37; Sidorov, '37; Gins- 

 berg, '39; Schmalhausen, '40). A saline 

 mediimi is so unfavorable an environment 

 that a vesicle does not form from neurular 

 ear ectoderm of anurans (Guareschi, '35). 



Its differentiation also can be disturbed by 

 ultraviolet radiation (Diirken, '51). A pro- 

 gressive differentiation occurs during com- 

 parable stages of the chick (Waddington, 

 '37; Stcherbatov, '38; Waterman and Evans, 

 '40; Levi-Montalcini, '46). Sensory areas be- 

 come self-differentiating in the chick at an 

 early stage and their development does not 

 depend upon the presence of nerve fibers 

 (Evans, '43). 



Shortly before the ear plate has formed, 

 the ear ectoderm has characteristics of an 

 equipotential system in that a whole ear can 

 form from a part and a single normal ear 

 from two fused rudiments (Harrison, '24). 

 After the vesicle has formed, this ability is 

 lost and the vesicle becomes a mosaic (Kaan, 

 '26; Guareschi, '30). From observations on 

 normal and experimental embryos the defini- 

 tive parts of the labyrinth can be related 

 to parts of the ear plate (Norris, 1892; Kaan, 

 '26). The endolymphatic duct and sac along 

 with the macula of the saccule arise from 

 the dorsal half, mainly the dorsoposterior 

 quadrant. The utricle and its associated 

 canals arise from the ventral half. 



Although the ear ectoderm has started to 

 differentiate during gastrulation, the ecto- 

 derm about the ear rudiment and the under- 

 lying tissues form a field capable of recon- 

 stituting a more or less normal labyrinth 

 until the ear rudiment itself is a mosaic. 

 This stands in contrast to the rapid loss of 

 ability of prospective epidermis to form neu- 

 ral plate during the end of gastrulation. 

 Regenerative capacity following extirpation 

 of the ear rudiment is gradually lost and 

 it also varies inversely with the amount of 

 ectoderm removed (Kaan, '26). The relative 

 ability of foreign ectoderm to form an ear 

 in the late neurula decreases with increase 

 in distance from the ear region in a given 

 direction; the most competent ectoderm at 

 this stage lies ventral to the ear region, the 

 least competent anterior to the ear region 

 (Yntema, '33). 



Even after the ear rudiment has become 



415 



