424 



Special Vertebrate Organogenesis 



tion of the neural folds and any intimate re- 

 lation between nasal and neural rudiments 

 (Carpenter, '37; Zwilling, '40; Schmal- 

 hausen, '50). 



A second induction by the neural crest and 

 fold adjacent to the nasal rudiment has been 

 indicated by several types of observations: 



(1) the typical association of the nasal 

 placode with the forebrain in cyclopia (e.g., 

 Adelmann, '37) and in experiments pri- 

 marily concerned with neural inductions 

 (Raven, '33; Mangold, '33b; Holtfreter, '36); 



(2) dependence for differentiation upon pres- 

 ence of the underlying neural fold or fore- 

 brain rudiment in heterotopic positions 

 (Luna, '15; Siggia, '36; Kawakami, '41) and 

 in explants (Mangold, '33a); (3) induction 

 from prospective gastrular ectoderm by neu- 

 ral tissue (Woerdeman, '38); (4) induction 

 by heterotopic forebrain rudiment from flank 

 ectoderm (Zwilling, '34; Kucherova, '45; 

 Kawakami, '36). Zwilling ('40), however, 

 has carefully repeated this last experiment 

 xenoplastically with negative results and 

 ascribes the positive reports to inclusion of 

 the nasal rudiment with the transplant. It 

 may be noted that regeneration of the nasal 

 organ occurs along with reconstitution of the 

 forebrain in the chick (Waddington and 

 Cohen, '36), and that the regeneration was 

 found by Luna ('15) to be dependent upon 

 presence of adjacent neural rudiments in the 

 frog. The results of Siggia ('36) indicated pre- 

 viously contradict this observation of Luna. 



The evidence indicates a series of two 

 inductions of the nasal rudiment, but clari- 

 fication of the problem is obviously needed. 



The normal inductors are not species spe- 

 cific in Amphibia (Holtfreter, '35; Zwilling, 

 '40). As in the case of the ear, abnormal in- 

 ductors may bring about differentiation of 

 a nasal rudiment without accompanying 

 brain tissue. These include the eye rudiment 

 with or without adjacent neural ectoderm 

 (Ikeda, '37; Woerdeman, '38), heated mouse 

 liver (Holtfreter, '34) and alcohol-treated 

 guinea pig thymus (Toivonen, '40). 



FORMATION OF SENSORY EPITHELIUM 

 AND NASAL PASSAGEWAYS 



The olfactory ectoderm and its derivative, 

 the olfactory nerve, can differentiate with 

 no central nervous connection in Amphibia 

 (Bell, '06; Siggia, '38; Cooper, '43) and in 

 the chick (Street, '37). Fibers of the olfactory 

 nerve have a proliferative effect on the region 

 of the brain which they may enter in both 

 normal and heterotopic locations (Burr, '24a, 



24b; May, '27). Shaping of the ectodermal 

 nasal passageways in heterotopic transplants 

 depends upon presence of the mesectoderm 

 siurrounding the nasal pit (Cooper, '43). 

 Moreover, most of the ectodermal nasal 

 structures can form without nearby ento- 

 derm. In the chick epithelia and cartilage 

 can differentiate independently but their 

 parts are more normal if they interact 

 (Street, '37). 



FORMATION OF NASAL CARTILAGES 



The dependence of the nasal cartilages 

 upon the nasal sac in urodeles has been 

 described in contradictory fashion by Burr 

 ('16) and Schmalhausen ('39). According 

 to the earlier work, the cartilages differenti- 

 ate independently of the sac. In absence of 

 the sac they collapse and join the rudiment 

 of the trabecula, thereby enlarging it. In the 

 latter finding the olfactory cartilages proper, 

 that is, the capsule exclusive of its medio- 

 basal part, form only in presence of the nasal 

 sac and arise in part from mesectoderm de- 

 rived from the sac itself. The size of the 

 trabecula is not increased following extirpa- 

 tion of the nasal rudiment but varies with 

 the size of the adjacent brain. These findings 

 are contradictory and call for further investi- 

 gation. The report of Burr recalls the in- 

 dependent formation of the ear cartilage in 

 Acipenser; the report of Schmalhausen in- 

 dicates an inductive action of the nasal sac 

 on mesenchyme such as the amphibian 

 auditory vesicle exerts. 



PROBLEMS 



Many of the general problems of embryol- 

 ogy can be applied to the ear and nose. The 

 nature of organization can be studied in a 

 system apart from neural induction. Work on 

 the specificity of inductors and the inductive 

 effects of chemical compounds has only been 

 initiated. The factors which produce struc- 

 tural specialization remain imknown; in re- 

 gard to the ear, the question exists as to why 

 a crista forms in one part, a macula in 

 another. Polarization of ectoderm in relation 

 to intracellular organization has been dis- 

 cussed but a means of analysis is as yet un- 

 known. 



An outstanding lack in studies on the ear 

 and nose is work on the structural basis of 

 the origin of function. Some correlation 

 would be desirable between the structure 

 and composition of the sense organs at vari- 

 ous stages and the functional derivatives of 



