268 



THE MOSAIC STAGE OF DIFFERENTIATION 



also produce glomeruli, lo-hour pieces differentiate a Wolffian duct ; 

 older pieces give complete self- differentiation of the mesonephros. 

 Feather-buds present the same picture.^ 



Other experiments have been performed in which the pieces cut 

 from the blastoderm were larger, representing one-third instead 

 of one-fifth of the whole area. While further results are desirable, 

 it seems from those already obtained that these larger pieces differ- 

 entiate more fully than the smaller ones, thus indicating that the 

 size of the piece is also a factor in the capacity of tissues to undergo 

 histological differentiation. ^ 



It will be remembered (see Chap, vi, p. i6o) that in the chick 

 blastoderm there is a gradient of developmental potencies at differ- 

 ent levels along the axis, and this must be taken into account in 

 interpreting the results of grafts of portions of blastoderms.^ 



A similar gradient of potency for differentiation has been noted 

 in the case of portions (thirds) of ii-day rat embryos grafted on to 

 the chorio-allantoic membrane of chicks, and can be stated in 

 tabular form.^ 



These cases in the rat are perhaps hardly comparable to those in 

 the chick, owing to the difference in age of the fragments tested. 

 Until further information is obtainable concerning the existence 

 and possible spread of a labile determination in these forms, it is 

 hazardous to attempt an interpretation of these cases. 



Hoadley, 1924, 1925. 



Willier and Rawles, 193 1 b; Hunt, 



[932. 



2 Murray and Selby, 1930. 

 ^ Hiraiwa, 1927. 



