Heart, Blood Vessels, Blood, and Entodermal Derivatives 



441 



Entoderm 



Fig. 157. Location of presumptive heart material in Amblystoma. A, Organ-forming areas mapped on the 

 ventral portion of an opened gastrula, dorsal view; B, gastrula, lateral view; C, neurula, stage 15, right 

 lateral view; D, stage 22. Heart-forming areas on C and D are outlined by broken line. E, transverse section 

 at level of arrow in D; F, enlargement of ventral portion of same section. (A and B, redrawn after Holtfreter, 

 '38; C, redrawn after Bacon, '45.) 



shows the location of bilateral areas of an 

 early chick blastoderm which have the po- 

 tency for differentiating into typical cardiac 

 muscle when divided into small pieces and 

 transplanted to the chorioallantoic mem- 

 brane of older host embryos. The grafts 

 which contained parts of the areas shown by 

 the darkest shading in Figure 159 produced 

 heart muscle with the greatest frequency. 



Heart-Forming Field and Induction. From 

 the experiments cited above and from others 

 which we will consider now it is evident 

 that the heart resembles other organs in the 

 sense that its field is more extensive than its 

 presumptive material. It has been shown for 



both anurans (Ekman, '21) and urodeles 

 (Copenhaver, '26) that a normal heart can 

 develop after the complete extirpation of a 

 visibly demarcated heart primordium (Fig. 

 158). In Amblystoma, the heart-forming po- 

 tency of the neighboring mesoderm is lost 

 in late tail-bud stage embryos (after Harri- 

 son's stage 29); in Bombinator, it is lost 

 earlier in correlation with the relatively 

 earlier heart differentiation in anurans. 



The extent of mesoderm with heart-form- 

 ing potency at the stage when the heart 

 primordia are first visibly indicated has been 

 studied by Ekman ('25). He found the meso- 

 derm from the gill region can be induced to 



