THE MOSAIC STAGE OF DIFFERENTIATION 243 



produce large but properly proportioned nervous systems; there 

 may, however, in some cases, be a certain amount of duplication of 

 the extreme anterior end (fig. 118). 



In Urodeles, removal at the early neurula stage of a portion 

 of the region of the presumptive eye-rudiment does not prevent 

 the embryo from developing normally as regards its brain and 

 paired eyes^ (fig. 119). 



All these experiments show that the neural fold region is deter- 

 mined as a whole at these stages : within the neural fold region there 

 also appears to be a determination along the antero-posterior axis 

 of the levels of the various constituent subregions, much as in the 

 limb. There is, however, no evidence that the neural fold region as a 

 whole ever passes through a totipotent phase in which any part of 

 sufficient size can regulate to produce a whole, as does the limb- 

 field. 



It is to be noticed in the experiments mentioned above, in which 

 the presumptive neural fold region was removed altogether in the 

 trunk region and only reduced in amount in the head, that regula- 

 tion takes place within levels on the antero-posterior axis of the 

 embryo, but not along that axis. The neural fold tissue of the anterior 

 region regulates to form a brain, level for level, but it does not 

 regulate longitudinally to form brain and spinal cord, i.e. structures 

 characteristic of other levels. 



As with the limb, ear, gill, and heart-fields, the nervous system 

 is polarised. The existence of this polarisation or gradient is shown 

 by the following experiment. If at the early neurula stage in an 

 embryo of Amhlystoma one presumptive eye-region is cut out, 

 rotated through 90° and grafted back again, the resulting embryo 

 possesses on the operated side an eye-cup which is subnormal and 

 deficient in its development and differentiation. Had this region 

 been extirpated completely, the remainder of the neural fold field 

 would have regulated to give rise to perfect eyes (see above). It 

 follows therefore that the region in question possesses a polarity 

 which at this stage presents obstacles to regulation if it is interfered 

 with. 2 The diflFerence from such a field as that of the limb is that 

 here the chemo-differentiation of subregions at diiferent levels 

 occurs at the first formation of the field, instead of later. 



^ Adelmann, 1929, 1930; Mangold, 193 1 A. ^ Woerdeman, 1932. 



16-2 



