THE MOSAIC STAGE OF DIFFERENTIATION 



229 



Neural Tube Field 



Fig. 112 

 Diagram of an amphibian neurula to 

 show the approximate localisation of 

 the main regional fields as yet dis- 

 covered by experimental analysis. The 



ism in the pre-mosaic stage, in combining a general determination 

 with an epigenetic mode of development. 



The arm-field and the leg-field are each of them self-differenti- 

 ating in a general way, and will produce only an arm or a leg, as the 

 case may be.^ But certain details of the development of the field 

 are not independent of the im- 

 mediate environment, i.e. the re- 

 mainder of the organism, and 

 in particular, its gradient-field. 

 We have seen that the arm-field 

 of Amblystoma is polarised from 

 its inception. The leg-field also 

 has some polarity from the start, 

 as is revealed when it is grafted 

 heterotopically in an abnormal 

 orientation: the original antero- 

 posterior axis of polarity of the 



disc persists and becomes that of arrows indicate that the fields are 

 the limb. After rotation of the known to be polarised from their first 

 . . appearance. (Origmal.) 



leg-disc at the origmal site (ortho- 

 topic), however, it acquires a new polarity in relation to that of the 

 body as a whole, i.e. the leg-disc has a new antero-posterior axis 

 impressed upon it.^ It may be hazarded that this result is due to the 

 size of the disc rotated not being large enough to cover the whole 

 limb-field. Later, however, the antero-posterior axis of the leg-disc 

 is entirely fixed, but the dorso-ventral axis is not. This is also the 

 condition of the arm-disc at the earliest stage studied (middle 

 gastrula). Rotation experiments with discs at this stage show that 

 the determination of the dorso-ventral axis is dependent on the body 

 of the host. Later still, the dorso-ventral axis of the discs is also 

 determined, and the limb-disc has by then proceeded far on the 

 way to becoming a mosaic of determined subregions. 



The fact that a field, although qualitatively determined, is capable 

 of being quantitatively influenced in its development, is well shown 

 by those experiments in which an ^arm-disc of an Amblystoma 

 neurula is grafted in place of an extirpated leg-disc. The graft 



^ Harrison, 1918; Ruud, 1929, 193 1 1 Stultz, 193 1. 

 2 Stultz, 193 1. 



