THE MOSAIC STAGE OF DIFFERENTIATION 251 



of the embryo from passing through this early phase of morpho- 

 logical differentiation (mass movements at gastrulation and neuru- 

 lation), and then to see whether it is capable of undergoing 

 histological differentiation.^ 



It will be remembered that during gastrulation in Amphibia, the 

 presumptive neural fold material undergoes a translocation in a 

 particular direction for each piece of tissue, so that the material is 

 brought into position for the formation of the neural folds in the 

 neurula (see p. 25). A piece of presumptive neural fold tissue may 

 be grafted into the dorso-lateral region of another embryo in the 

 gastrula stage and orientated in such a way that the movements 

 of the host tissues in which it becomes involved are either directed 

 parallel or perpendicular to the direction in which the tissue would 

 have moved had it been left intact in situ. It is found that the 

 tissue differentiates morphologically into neural folds regardless 

 of its orientation and of the direction of the movements which 

 it has undergone. 2 



It can be concluded from these experiments that specific form- 

 changes are not necessary for subsequent histological differentiation. 

 Other recent investigations of the histological differentiation of the 

 cell-regions in early embryonic stages of Triton have shown that 

 certain histological distinctions between presumptive epidermis and 

 presumptive neural fold are independent of form-changes. These 

 distinctions are already present at the earliest neurula stage. The 

 cells of the neural fold region are elongated, arranged in a single 

 layer, and have ellipsoidal nuclei ; their pigment is concentrated at 

 the outer end of the cells. The cells of the epidermal region are 

 cubical, arranged in two layers; the nuclei are spherical, and the 

 pigment is distributed irregularly. 



If at the neurula or late gastrula stages (i.e. after the organiser has 

 been invaginated and underlies the presumptive neural folds) 

 pieces of presumptive neural fold tissue or presumptive epidermis 

 are grafted into atypical positions, they develop, as we have already 

 seen, by self-differentiation, and undergo the histological differ- 



^ Goerttler, 1927. It may be noticed, however, that this experiment results 

 not only in forcing the piece of tissue to undergo abnormal movements, but it 

 also interferes with its polarity, which, as we have already seen (p. 243), plays 

 an important part in differentiation. 



2 Holtfreter, 1933 a. 



