178 



PRINCIPLES OF EMBRYOLOGY 



that from which it would normally develop (Fig. 10.3). The remainder 

 of the egg had much less inherent capacity. In particular, the presumptive 

 neural plate and the presumptive epidermis were alike in that when 



Figure 10.3 



The urodele gastrula seen from the side; showing A, the presumptive fate 

 of the areas (what they will normally develop into); B, what they will self- 

 differentiate into when small parts are isolated in saline ; C, the 'prospective 

 potency', i.e. what they can be induced to develop into; D, the region of 

 the primary organisation centre ('head' organiser, dots; 'tail' organiser, 

 dashes). (From Holtfreter.) 



isolated they both formed merely a generalised epidermal tissue with no 

 special differentiations. This region of the gastrula must not, however, 

 be considered as completely neutral and characterless. It has one most 

 important property; namely the readiness to react to the organiser stimu- 

 lus. This property shows no sign of being in existence before the onset of 



