416 



Special Vertebrate Organogenesis 



STACClJ DORSAL View 



SrACCIS ANTCRiCRviEW STACCIS DORSAL View 



Fig. 145. Relative positions of ectodermal rudiments of the head in neurular stages of Amblystoma 

 punctatum (Carpenter, '37; Harrison, '45). 



■ — SOMITE 

 O— ENTODER 



SR-SINUS » / \ •/ . *• 



RHOMBOIDALISX / \' / ▼"HEART 



— NEURAL CRESt\ \_y ^ ■/' ©-CHORDA 



LP-LATERAL PLATE \^ /• 0-NEPHROS 



-HEAD MESODERM ^«v^ >/ •-ERYTHROCYTES 



Fig. 146. Map of prospective areas in the primitive 

 streak blastoderm of the chick (Rudnick, '44). 



a mosaic its differentiation may be adversely 

 affected by relationship with parts of the 

 central nervous system other than the region 

 with which it is normally related (Detwiler 

 and Van Dyke, '50; Detwiler, '51). It has 

 been concluded that the medulla exerts 

 a late morphogenetic influence on the otic 

 vesicle. However, it can be shown that the 

 medulla is not necessary for late differentia- 

 tion. If the hindbrain is removed at an ear 

 plate stage a normal labyrinth forms 

 (Yntema, unpublished). Mechanical factors 

 are of importance, especially the formation 

 of a capsule (Kaan, '38) and a functional 

 endolymphatic sac which may regulate hy- 

 drostatic pressure of the endolymph (Harri- 

 son, '36a). However, presence of capsule 

 and of endolymphatic sac do not in them- 

 selves assure normal development (Detwiler 

 and Van Dyke, '50). The size of the ear in 

 the salamander influences the number of 

 cells in the acoustic area of the medulla 

 (Richardson, '32), and absence of the laby- 

 rinth in the chick results in less marked dif- 

 ferentiation of some associated medullary 

 nuclei (Levi-Montalcini, '49). Neurons of 

 the auditory ganglion are derivatives of the 

 wall of the auditory vesicle (Yntema, '37); 

 hence they serve as an index of differentia- 



