organisers: inducers of differentiation 



H3 



possesses a ventral sucker, although the organiser which induced it 

 comes from a species which does not possess one.^ As the matter 

 has been figuratively put : the organiser disposes of the fates of the 





h. 



Fig. 66 



The preservation of specific characters by a tissue, in spite of its having been in- 

 duced to undergo differentiation into structures other than those representing its 

 presumptive fate, a, A piece of ventral epidermis from a gastrula of the frog Rana 

 esculenta is grafted into the mouth- region of an embryo of the newt Triton 

 tae?iiatus, where it differentiates into mouth-parts in accordance with its position, 

 but, in addition, gives rise to ventral suckers (h., h.). b, Section through such an 

 embryo, showing: b. basal membrane of grafted epidermis; sc. typical secreting 

 cells of ventral sucker; 5. functional secretion. A sucker is never formed by a 

 newt. (From Spemann and Schotte, Natiiriuiss. xx, 1932.) 



tissues in a general way, but as regards the details of their differen- 

 tiation, the tissues already possess thieir instructions.- 



The age at which an organiser first acquires its power of in- 

 duction is not known, but constriction experiments on the egg of 



^ Spemann, 1932, 1933; Spemann and Schotte, 1932. - Spemann, 1921. 



