i8o organisers: inducers of differentiation 



stomodaeal ectoderm.^ [See also p. 498.] The perforation of the 

 choanae, on the other hand, is dependent on the estabhshment of 

 contact between the nasal rudiment and the endodermal roof of 

 the mouth. Even a rudimentary nasal pit is capable, provided it 

 establishes contact with the endoderm, of inducing the latter to give 

 rise to a typically normal choana.^ 



A curious case is that of the perforation of the operculum in 

 Anuran larvae at metamorphosis. This occurs on the right-hand 

 side, allowing the right fore-limb to emerge (the left fore-limb 

 emerges through the spiracle, an aperture which has been present 

 since the first formation of the operculum). It was at first supposed 

 that this was due to mechanical pressure exerted by the growing 

 limb. Then it was discovered that perforation took place even if 

 the rudiment of the fore-limb had previously beeen extirpated.^ 

 Finally, it has been established that the perforation is caused by a 

 substance produced by the gills as they degenerate during meta- 

 morphosis.* The degenerating gills will cause perforation of the 

 skin in any region if grafted beneath the surface (see p. 429 and 

 figs. 208, 209). 



In Echinoderms, it has been shown that the formation of the 

 amnion and of large portions of the rudiment of the adult sea- 

 urchin are dependent on the presence of the hydrocoel. This follows 

 from the cases in which the abnormal presence of a right hydrocoel 

 is accompanied by the formation of a right amnion and echinoid 

 rudiment, with dental sacs, perihaemal rudiments, oesophagus, and 

 mouth. ^ The size of this echinoid rudiment is correlated with that 

 of the hydrocoel;^ and in those cases in which by experimental 

 treatment the position of the left (normal) hydrocoel is altered, it 

 is found that the amnion and adult echinoid rudiment arise im- 

 mediately over the hydrocoel wherever it happens to be, and not 

 from their presumptive tissues.^ At the same time, it seems that 

 the presence of the amnion is necessary for the complete diflFerentia- 

 tion of the hydrocoel, so that we are here confronted with a case of 

 mutual dependence.^ 



Another example of this, also from Echinoderms, concerns the 



^ Adams, 1924, 193 1. " Ekman, 1923. 



3 Braus, 1906. ^ Helff, 1924, 1926. 



^ MacBride, 1911, 1918. ^ von Ubisch, 1913. 



^ Runnstrom, 1918. ^ Runnstrom, 1929. 



