422 



Special Vertebrate Organogenesis 



Fiirther characteristics o£ induction of 

 the ear have been obtained by replacing ear 

 ectoderm by foreign ectoderm; grafts of 

 prospective gill ectoderm from various stages 

 are placed in the ear region of various 

 stages of Amblystoma (Yntema, '50). The 

 relative responses are represented in a three- 

 dimensional graph (Figs. 150 and 151). 

 Mesodermal activation is found to be great- 

 est in the early neural plate stage and to 

 diminish rapidly during neurulation. Neural 

 activation appears during late neurulation 

 and is greatest at the time of closure of 

 the neural folds. It persists longer than the 

 mesodermal and indications of it are present 

 in motile stages. Periods of maximal re- 

 sponse occur in the foreign ectoderm; these 

 precede by short intervals the respective 

 periods of maximal activation. In Am- 

 blystoma, foreign ectoderm of the early neu- 

 rula, competent to respond to mesodermal 

 activation, is not as yet competent to respond 

 to neural activation. This second competence 

 is acquired during neurulation. Both com- 

 petences, once acquired, are retained for a 

 considerable period in decreasing intensity. 

 The observations that the first competence 

 is qualitatively different from the second 

 and that ectoderm with only the first com- 

 petence does not respond to the second or 

 neural activation show that the two acti- 

 vations, mesodermal and neviral, are also 

 qualitatively different. In anurans, the com- 

 petence to respond to neviral activation is 

 present in prospective ectoderm of gastrular 

 stages (Ponomarewa, '38; Schmidt, '38). 



The relative importance of the two types 

 of inductions for normal development varies 

 among different groups of amphibians (Gins- 

 berg, '46, '50). The normality of ears result- 

 ing from mesodermal induction only and 

 the increments of response brought about 

 by neural activation can be demonstrated by 

 removing neural plate and fold related to 

 the ear ectoderm before and during neural 

 induction in Amblystoma (Yntema, unpub- 

 lished). Mesodermal induction alone results 

 in imperfect small ears. By the time the ear 

 plate has formed the rudiment is able to 

 develop into a normal labyrinth without 

 further activation by neviral tube or crest. 



FORMATION OF CAPSULE OF INNER 

 EAR 



Development of a cartilaginous capsule for 

 the membranous labyrinth of the inner ear 

 depends upon the presence of the latter 

 (Lewis, '07), and the normality of the mem- 



branous labyrinth depends upon presence 

 of the capsule according to Kaan ('38). This 

 latter relationship has been questioned by 

 Detwiler and Van Dyke ('50). In absence 

 of the auditory vesicle, capsular cartilage 

 does not differentiate in amphibia (Lewis, 

 '07) and in the chick (Reagan, '17; Yntema, 

 '44). In a fish, Acipenser, a solid cartilag- 

 inous body forms in the ear region after 

 extirpation of the vesicle (Filatow, '30). 

 In the former cases the vesicle induces the 

 mesenchymal cells to form cartilage. In 

 the latter case the differentiation of cartilage 

 is doubly assured since it occurs in absence 

 of the vesicle and since the vesicle can in- 

 duce a capsule heterotopically. 



The dependence of capsule formation has 

 been further analyzed by heterotopic and 

 heteroplastic transplantations of the vesicles. 

 Following heteroplastic exchange of audi- 

 tory vesicles, the amount of capsular car- 

 tilage formed is influenced by the nature of 

 both the graft and the host, e.g., Amblystoma 

 punctatum. and A. tigrinum (Richardson, 

 '32). In all Amphibia tested a heterotopic 

 vesicle near the ear region will induce a 

 more or less complete capsule (Luther, '24) 

 and the induction is interspecific, since an 

 anuran vesicle will induce capsule from the 

 tissue of a urodele host (Lewis, '07). Re- 

 ports as to the ability of the ear vesicle 

 to induce capsule formation in the trunk 

 are contradictory. According to most stud- 

 ies on urodeles, no capsular cartilage de- 

 velops except in some instances in which 

 the vesicle lies next to a developing vertebra 

 and apparently induces hyperplasia of the 

 vertebra (Balinsky, '25; Yntema, '33). In 

 anurans a capsvile frequently appears but 

 from no such apparent source (Filatow, '27). 

 In xenoplastic combinations the cartilage 

 about anuran labyrinths in urodele flanks 

 is anuran without contribution from the host 

 (Balinsky, '27; Kaan, '30) and so has arisen 

 from the graft, though care was taken by 

 Kaan to exclude mesectoderm of the neural 

 crest and mesentoderm. Heterotopic vesicles 

 on the head of the urodele have capsules 

 which are derived in part from the host and 

 in part from the grafted ear ectoderm 

 (Yntema, '39). These results indicate the 

 conclusion that the ear rudiment or the 

 ectoderm surrounding it may be a source 

 of mesectoderm for the ear capsule, at least 

 in the heterotopic position. This is contrary 

 to the view generally held that the capsule 

 arises from mesentoderm alone both in the 

 normal position (e.g.. Stone, '26) and 

 heterotopically (e.g., Kaan, '30). The mes- 



