organisers: inducers of differentiation 177 



between the nasal sac and olfactory capsule described above. After 

 removal of the ear-vesicle in Acipe?iser, no auditory capsule is 

 formed, but a shapeless chondrification appears in its place. Here, 

 then, the actual formation of cartilage is independent of the ear- 

 vesicle, but the differentiation of the cartilage into an auditory 

 capsule is dependent.^ 



Of a different nature again is the response of the uterine mucosa 

 to the presence of foreign bodies in the uterus.'^ Any foreign body — 

 glass, platinum wire, paraffin, etc. — causes a proliferation of the 

 mucosa essentially similar to that w^hich it shows as a result of the 

 implantation of the embryo. In both cases, the proliferation will 

 only occur provided that certain of the ovarian hormones are present 

 in the blood-stream. Here, the response is not the direct result of 

 mechanical forces, as with the arms of the pluteus. A somewhat 

 similar case from the early stages of development is the effect of 

 grafting foreign objects under the flank ectoderm of Urodele em- 

 bryos : these in some cases induce the formation of supernumerary 

 limbs. These experiments w^ere first performed with ear-vesicles,^ 

 but it has since been found that inorganic objects, such as celloidin 

 beads, have the same effecf* (see also Chap, x, p. 362, for the 

 effect of nerve- endings on limb- induction). The type of structure 

 induced thus appears to be determined by local regional factors, 

 regardless of the specific nature of the graft, which acts as a 

 releasing mechanism (see Chap, vii, p. 231). 



Another case which appears to be comparable is the induction of 

 supernumerary balancers in Triton within the balancer field (see 

 p. 236) by means of grafts of neural crest cells of Rana,'' or of 

 anterior neural plate cells, or even of fore-gut wall-cells of Ambly- 

 stoma tigri?tum, which donors possess no balancer.^ These cases 

 serve as a further illustration of the fact noted above (p. 140), that 

 local regional properties of the tissues acted upon, as well as the 

 properties of the releasing mechanism, do play a part in deter- 

 mining the quality of the induced structure. The relative import- 

 ance of these two sets of factors varies in different cases: the 

 amphibian organiser is capable of overriding nearly all the local 



^ Filatow, 1930. 2 L Loeb, 1908. 



^ Balinsky, 1925-6; Filatow, 1927. ^ Balinsky, 1927. 



'" Raven, 1931A. ^ Mangold, 1931c. 



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