420 



Special Vertebrate Organogenesis 



tures along the two main axes of the laby- 

 rinth may act both before and after the 

 critical stages for the axes indicated by 

 Harrison. The AP polarity of the ear ecto- 

 derm appears to differentiate gradually 

 along with other characteristics of the ear 

 ectoderm. 



Some studies on the ciliary beat of the 

 ectoderm within the ear vesicle bear on this 

 subject (Woerdeman, '41), The direction of 

 the beat is already fixed in the presumptive 

 ectoderm of the Triton gastrixla, although 

 the stages for polarization of the main axes 

 of the labyrinth are comparable with those 

 of Amblystoma. Hence a polarity is demon- 

 strable in the ectodermal region of the gas- 

 trula. The polarization of the ear rudiment 

 may be a local expression of a general po- 

 larity: this is indicated by some experiments 

 on Amblystoma (Yntema, '48) in which 

 prospective gill ectoderm was placed apdd 

 (anteroposterior axis inverted, dorsoventral 

 axis not inverted) into the ear region, using 

 donors and hosts shortly before and after 

 the end of neurulation. As a result the for- 

 eign ectoderm may form disharmonic or 

 reduplicated labyrinths. Under these condi- 

 tions polarization and induction are sep- 

 arable processes. Fixation of polarity of the 

 ciliary beat precedes that of the ear vesicle, 

 as Woerdeman and others have shown. From 

 this Woerdeman concluded that the proc- 

 esses of polarization were not the same for 

 the two systems. It is also possible that 

 polarities of various organs, fixed at different 

 stages, are differential derivatives of a basic 

 but labile pattern in the cells involved. In 

 such a system one derivative of polarity 

 might be fixed, the basic polarity then re- 

 versed, and a second derivative of the same 

 cells would then be disharmonic with the 

 first. 



The amphibian ear vesicle has been ro- 

 tated experimentally after it has become 

 a mosaic and its axes fixed (Spemann, '10; 

 Streeter, '07, '14; Ogawa, '21, '26; Tokura, 

 '24, '25; Hall, '37). Under some conditions 

 the vesicle develops in its new position; 

 under others it tends to tiu-n back to its 

 normal position, either partially or com- 

 pletely. The factors which produce such a 

 turning remain undetermined; Streeter ('14, 

 '21) has suggested some possibilities. 



INDUCTION OF THE AUDITORY 

 VESICLE 



As Harrison ('35, '38) has pointed out, 

 the formation of a normal ear results from 



the interplay of several factors: the position 

 of the rudiment, as well as ectoderm, meso- 

 derm, and myelencephalon, is involved. 

 Two relations of tissues are to be noted 

 especially: (1) that chordamesoderm comes 

 to lie under the prospective ear ectoderm 

 during gastrulation, and (2) that the raising 

 of the neural folds brings the hindbrain 

 rudiment in relation to the ear ectoderm 

 (Yntema, '46; Ginsberg, '46). 



A mesodermal inductor, implied by some 

 earlier results (e.g., Dalcq, '33; Holtfreter, 

 '33), was demonstrated in Amblystoma by 

 Harrison ('35, '38, '45). Belly ectoderm is 

 grafted in place of the neural plate and 

 folds of the hindbrain; subsequently two 

 vesicles form on each side, a lateral one 

 from the presumptive ear ectoderm, a medial 

 one from the graft overlying the chorda- 

 mesoderm which has served as the inductor. 

 Induction by mesoderm of Rana has been 

 reported by other workers (Albavun and 

 Nestler, '37; Zwilling, '41), and the ob- 

 servations of Kogan ('44) on Triton neu- 

 rulae indicate that chorda of the hindbrain 

 level is an active inductor. However, when 

 chorda from the Triton neurula is trans- 

 planted to the gastrula, posterior chorda may 

 induce ear formation in the trunk of the 

 host (Borghese, '42). Ability to induce ear 

 is spread throughout the roof of the archen- 

 teron but it is greatest at the anterior level 

 of the notochord; heat destroys this capacity. 

 These same cells while still part of the 

 dorsal lip of the blastopore can induce ear. 

 Even the uninvaginated mesoderm on the 

 posterior neural plate of the early neurula 

 retains this ability in Triturus (Kawakami, 

 '43, '49). Development of vesicle in absence 

 of adjacent brain in the chick (Szepsenwol, 

 '33), in Discoglossus (Pasteels, '39), or after 

 its early removal in the chick (Levi-Montal- 

 cini, '46), and studies on Rana hybrids by 

 Moore ('46) indicate that paired mesodermal 

 inductor regions are localized in the pri- 

 mary organizer of the early gastrula. The 

 induction of the ear begins as a part of 

 the primary organization occurring during 

 gastrulation and continues through later 

 stages. 



The beginning of the second period of 

 induction is characterized by the approxima- 

 tion of the presumptive ear ectoderm to the 

 adjacent neural fold. The inductive capacity 

 of the hindbrain rudiment has been demon- 

 strated by the formation of the ear vesicle 

 from ectoderm next to a heterotopic hind- 

 brain in Amblystoma (Stone, '31), and in 

 Triton, Rana and Bombinator (Gorbunova, 



