244 THE MOSAIC STAGE OF DIFFERENTIATION 



Progressive chemo-differentiation within the neural fold field 

 may best be illustrated with reference to the eye-region. As we 

 have seen (p. 243), experiments involving extirpation of the pre- 

 sumptive eye-region or of part of it at the early neurula stage in 

 Triton and Amblystoma have shown that the eye-field is more ex- 

 tensive than the region which in normal development actually gives 

 rise to the eyes.^ Eye-forming potencies, as tested by grafting, 

 are higher in the mid-line than more laterally.^ The reason that 

 a single median eye is not normallyformed depends on the presence 

 of the underlying gut-roof (see below). Further, regulation takes 

 place most readily across the transverse axis of the neural fold 

 region.^ At the same time, experiments of grafting portions of the 

 eye-region into other parts of the body show that the eye-rudiment 

 is already invisibly chemo-differentiated.* If the entire eye-field is 

 extirpated, no eyes are developed.^ 



At the same time, although the eye-region is determined as a 

 subregion of the neural fold field at the neurula stage, it is still 

 capable of regulation within itself, as is shown by those experiments 

 on Amblystoma at the neurula stage in which a portion of pre- 

 sumptive eye-tissue is grafted into the belly wall and differentiates 

 there into a more or less well-formed eye consisting of tapetum and 

 retina. Curiously enough, these eyes lack a stalk, although the graft 

 included the region which would normally have given rise to the 

 optic stalk in the intact embryo. It follows, therefore, that the 

 various regions of the optic complex — retina, tapetum, and stalk- 

 are not rigidly determined within the eye-area at the stage in 

 question in Amblystoma.^ 



In Pelobates (Anuran) at the tail-bud stage, grafted portions 

 composed of tapetum only can regulate to form little optic cups 

 with retina and tapetum in correct proportions.^ The optic stalk, 

 however, at this stage is already predetermined. In other Amphibia 

 it has been found that two eye-rudiments, grafted together, regulate 

 to form one.'' 



A complication in the analysis of the progressive determination 



^ Woerdeman, 1929; Manchot, 1929. - Adelmann, 1930. 



^ Mangold, 1928, 1931A. 



^ Spemann, 1919; Spirito, 1928; Adelmann, 1929, 1930- 



■^ Adelmann, 1930; Stella, 1932. 



*' Dragomirow, 1932, 1933. " Pasquini, 1927. 



