234 



THE MOSAIC STAGE OF DIFFERENTIATION 



neighbouring regions : ^ it may therefore safely be concluded that 

 the heart-field is more extensive than the actual presumptive heart 



Fig. 115 

 Experiments on the development of the heart in Bombinator pachypiis . A, Single 

 normal heart formed from an enlarged rudiment, that of another embryo having 

 been added in the mid-ventral line : the parts arising from the graft shown dark. 

 B, C, Partially doubled hearts formed after grafting foreign tissue (pharyngeal 

 wall) into the mid-ventral line ; ventricle and bulbus show duplication ; the graft 

 has been used up in the formation of the heart in B, but in C a portion remains 

 as undifferentiated material. D, Heart duplication almost complete, as a result of 

 grafting a large piece of foreign tissue in the mid-ventral line; most of the graft 

 has remained undifferentiated, and only a small portion (shown dark) has con- 

 tributed to the heart in front and behind. E, Complete duplication as a result of 

 grafting a piece of foreign tissue which has remained undifferentiated ; the right- 

 hand member shows situs inversus. (Redrawn after Ekman, Arch. Entiomech. 

 cvi, 1925.) 



region. Heart-forming potencies decrease with increasing distance 

 from the normal presumptive heart region^ and at the neurula 



^ Ekman, 1921. " Ekman, 1925. 



