232 THE MOSAIC STAGE OF DIFFERENTIATION 



(3 J somites in diameter) were grafted after rotation, it was found 

 that the limb occasionally underwent a rotation at the shoulder- 

 girdle, so as to become correctly oriented. ^ When the disc was 

 rotated through an angle up to 235°, the limb might right itself by 

 a rotation in the reverse direction : on the other hand, if the disc 

 was rotated through 270°, the limb might complete the circle by 

 rotating the remaining 90' in the same direction.^ 



It appears that the rotation of the limb is in some ways dependent 

 on the shoulder-girdle. If the rotated disc is only i| somites in 

 diameter, it contains none of the girdle-rudiment, and no regu- 

 latory rotation takes place. In the case of rotated discs of the normal 

 diameter of 3 1 somites, parts of the shoulder-girdle are formed, and 

 regulatory rotation may take place. If the rotated disc is 5 somites 

 in diameter, a complete girdle is formed and the limb conforms to it, 

 without regulatory rotation.^ Lastly, if in a graft 5 somites in 

 diameter a 3J-somite disc is separated from a peripheral ring, and 

 then both central disc and peripheral ring are rotated independently, 

 the limb undergoes postural regulation with reference to the ring.* 

 Apparently, therefore, the portions of the girdle whose rudiments 

 lie outside the 3j-somite disc but within the 5 -somite ring, act as 

 determining factors on those portions of the girdle whose rudi- 

 ments are included within the 3J-somite disc. The girdle then 

 brings about the rotation of the limb, but in a manner which is still 

 obscure. 



§7 



Turning now to other examples of fields, we may take that of the 

 amphibian ear. This occupies a region of ectoderm on each side 

 of the head, behind the eye, and must in some way be dependent on 

 the organiser, since dorsal lip grafts are capable of inducing the 

 formation of ears. Here again, it is found that the field is more ex- 

 tensive than the normal presumptive rudiment, for if a portion of 

 the presumptive ear-area be removed at the early neurula stage in 

 Rana nigromaculatay a normal (though smaller) vesicle is formed, 

 and this can be shown to arise from the neighbouring cells which 

 have closed over the wound, though these would normally have 



^ Harrison, 1921 a. - Nicholas; 1924 b, 



2 Nicholas, 1926. * Nicholas, 1925. 



