THE MOSAIC STAGE OF DIFFERENTIATION 199 



§2 



This mosaic predetermination of various regions in the chemo- 

 differentiated stage in development is also demonstrated by 

 numerous experiments in which a region continues its presumptive 

 development even after grafting into an abnormal position. As 

 previously mentioned, the presumptive eye-region of amphibian 

 embryos has the power of self-differentiation at the early neurula 

 stage (p. 46). Limb-discs of Amhlystoma grafted on to the flank 

 or into other abnormal situations will still continue to form limbs. ^ 



The presumptive ear- region will differentiate semi-circular 

 canals, etc., when grafted into abnormal situations.'- The mosaic 

 nature of this power of self-differentiation is further shown by the 

 fact that if a neurula of Rana esculenta is divided transversely by a 

 cut passing through the presumptive ear-region, it is found that 

 both halves develop auditory vesicles, but they are incomplete, the 

 details varying with the precise position of the cut. On each side 

 of the body, there is only one ductus endolymphaticus developed 

 on each side, and this may be either in the anterior or the posterior 

 half.=^ 



Other experiments of grafting and extirpation have shown that 

 the gill-region, the balancer, nerve placodes, portions of the neural 

 crest, and various other amphibian organ-rudiments possess this 

 capacity for self-differentiation (for references, see later sections). 

 The outgrowth of the glomerulus from the aorta has been shown 

 to be due to self-differentiation, independent of the presence or 

 absence of the pronephros with which it normally comes into 

 functional relation.* 



In the chick, grafting of embryonic rudiments on to the chorio- 

 allantois of another embryo has been the main method employed, 

 e.g. with the ear-region, eye-region, complete limb-rudiments, 

 fractions of limb-rudiments, presumptive thyroid- region,^ meso- 

 nephros (see also below), metanephros, adrenal,^ spleen, portions 

 of brain and spinal cord,^ lung^, etc. (figs. 95, 96). 



^ Harrison, 19 18; Detwiler, 191 8. 



^ Streeter, 1906, 1907; Sternberg, 1924. 



^ Spemann, 1910. ^ Howland, 1916. 



^ Rudnick, 1932. ^ Willier, 1930. 



' Rienhoff, 1922; Danchakoff, 1924; Hoadley, 1924, 1925, 1926 a, 1929: 

 Murray and Huxley, 1925. ^ Rudnick, 1933. 



