organisers: inducers of differentiation 175 



position, so are the arms.^ It appears that the formative stimulus 

 consists in the continuous pressure exerted on the epidermis by 

 the growing tips of the skeletal spicules. But, as we shall shortly 

 see, the position of the skeletal spicules is itself under the control 

 of the epidermis, and therefore arms and spicules are, in a measure, 

 mutually dependent." 



Other examples of dependent differentiation are seen in the ad- 

 justment of the skeleton of Vertebrates to the underlying organs. 

 For example, if the rudiment of the optic-cup is extirpated in early 

 amphibian embryos, when the cartilaginous cranium comes to be 

 formed, the skeleton of the orbital region is markedly smaller on 

 the operated side, and, in certain respects, irregular."^ When foreign 

 structures, e.g. mesonephros, are grafted in place of the mid-brain, 

 the cartilaginous cranium is distorted by the increased intracranial 

 pressure due to the graft. ** When the rudiment of the nasal sac is 

 extirpated, the cartilages of the nasal region arise by self-differentia- 

 tion, but the nasal capsule is completely collapsed : the normal form 

 of the nasal capsule is attained through the cartilage adjusting its 

 growth to the form of the nasal sac.^ 



Of a rather different nature, however, is the relation of the carti- 

 laginous auditory capsule to the primary ear-vesicle. In this case, 

 the cartilaginous capsule wholly fails to develop if the vesicle has 

 been extirpated at an earlier stage. Conversely, a grafted ear- 

 vesicle may induce the formation of a cartilaginous capsule around 

 it. The dependence has been shown to obtain both in amphibian^ 

 and in avian^ embryos. Here it would appear that a chemical 

 stimulus from the ear-vesicle is necessary to initiate cartilage pro- 

 duction by the neighbouring mesenchyme, though, doubtless, 

 mechanical factors play a part in the later growth of the capsule. 

 The effect is not species-specific, for an ear-vesicle of Rana can 

 induce the formation of a cartilaginous capsule from tissues of 

 Amhly stoma, when grafted into an embryo of that animal.'^ 



In the fish Acipenser, the relations between the ear-vesicle and 

 the cartilaginous capsule are slightly different, but resemble those 



^ Herbst, 19 12. - Runnstrom, 1929. 



^ Steinitz, 1906. * Nicholas, 1930. 



^ Burr, 1916. 



•^ Filatow, 1916; Luther, 1925; Guareschi, 1928. 



' Reagan, 1917. ^ Lewis, 1906. 



