Ill SECONDARY AND MINOR INDUCTIONS 453 



enchyme forming the myoblasts of the heart. By vital staining of salamander 

 neurulae, Wilens (1955) has observed that the heart-forming mesenchyme is at first 

 located on both sides of the brain; when it migrates into the mesobranchial region', 

 it is replaced by another mesenchymatous group from nearl^y, which remains 

 in the otic region. This substitution, if confirmed, could well explain the 

 elusiveness of the primary inductor, and the obstinacy with which the otocyst 

 appears even in the rudimentary embryos. 



The action of the primary inductor is immediately relayed by a secondary one 

 represented by the lateral masses of the rhombencephalon^ (Yntema, 1 950) . It seems 

 also that the mesenchyme plays a role quite early, as suggested by the discussion on 

 p. 407. This influence may explain why the epithelium remains relatively thin and 

 insures the retention of the endolymph fluid, evidently analogous to the ependymal 

 liquid. These combined factors, however, account only for the appearance of the otic 

 vesicle as an almost spherical or ellipsoidal turgescent body, for the budding of the 

 ganglionic cells, and perhaps also for the first bulging of the endolymphatic canal. 



The main division into an utricular and a saccular part, the modelling of the 

 folds initiating the semicircular canals, the stretching and growth of the lagena 

 or cochlea, and many other more deUcate features lack an explanation. On the 

 basis of some too elementary cytochemical aspects concerning RNA and glycogen, 

 I suggested ( 1 953) that the vascular pattern may be instrumental in this modelling. 

 In the neighborhood of the young vessels, especially of the veins, it can be seen 

 that trophic substances are abundantly furnished to the mesenchyme and that, 

 with its cooperation, definite parts of the otocyst epithelium become more prosper- 

 ous and bulge or deform in the expected way. 



An elaborate investigation of the distribution of several metaboUtes and enzymes 

 in the cephalic mesenchyme and epithelial structures of mouse embryos has been 

 performed by Milaire. The pattern of glycogen distribution depends at first on 

 the dorsal aorta, later on the veins. The general picture substantiates my previous 

 hypothesis, but many details and complementary relations have now appeared^. 

 One experimental although indirect argument for this trophic epigenesis is that 

 in ectopic otocysts, as are formed in non-vascularized grafts of the embryonic 

 shield o{ Fundulus, the semi-circular canals do not appear (Oppenheimer, 1955). 

 This role of trophic epigenesis ought to be explored further experimentally*. 



The formation of the cartilaginous capsule of the ear is known to be a reaction 

 of true mesenchyme to the presence of the otic vesicle. Experiments of J. A. Benoit 

 {1955, -6, -7a, b) on chick embryos have confirmed this general statement by 

 showing that the mesenchyme does not react if the otocyst is replaced by a lens or 

 by a piece of paraffin. Moreover, they have revealed that all parts of the sur- 

 rounding mesenchyme do not have an equal tendency toward chondrogenesis. 

 These differences are probably related to the pattern of various territories which 

 cytochemical tests can detect in the mesenchyme of the head. 



^ The pharyngeal endoblast could be a secondary inductor for the heart (see Mangold, 1 956) . 



2 According to Nieuwkoop (1958) the neural crest of the rhombencephalon may by itself 



induce an otic placode. 



^ This paper will be published in Vol. 70 of the Arch. biol. (Liege). 



^ Walder (1950) has described avascular explants. 



Literalure f>. 483 



