442 



Special Vertebrate Organogenesis 



take part in heart formation when it is 

 transplanted into the mid-ventral area be- 

 tween the bilateral heart primordia, but 

 mesoderm from a more caudal region can 

 not. Ekman considered this an example of 

 true induction. His experiments pointed to 



tiation was not a factor in the results. The 

 evidence indicates the existence of a heart 

 inductor in the entoderm of the definitive 

 heart region. In further support of this view, 

 Bacon fovmd that indifferent presumptive 

 mesoderm from a beginning gastrula is in- 



V 



:?^ 





Mesoderm 





C Presumptive Presumptive 



Endocardium Myocardium 



Fig. 158. Heart localization in a tail-bud, stage 27, Amblystoma punctatum embryo. A and B, ventral and 

 lateral views respectively; C, ventral portion of a transverse section at level indicated on B. Extirpation of 

 all mesodermal tissue over areas outlined on A and B, extending from AT to F in section C, prevents heart 

 formation. Extirpation of presumptive heart material, extending from Z to W in section C, prevents heart 

 development only when the wound is filled with foreign tissue. Otherwise, mesoderm in XZ and YW areas 

 migrates ventrad and exhibits heart potency. 



the existence of an inductor in the heart 

 primordium, but other experiments have 

 shown that the inducing activity is not lim- 

 ited to the heart itself. It has been shown by 

 Bacon ('45) that a normally structured heart 

 can be organized when the heart rudiment 

 of tail-bud stage Amblystoma is replaced 

 with indifferent presumptive mesoderm from 

 the marginal zone of a beginning gastrula 

 (stage 10). The substituted tissue did not in- 

 clude any presumptive heart material (it did 

 not contain the lateral quadrants of the mar- 

 ginal zone) and therefore heart self-differen- 



duced to form cardiac tissue when it is grown 

 in cultixres with archenteron floor from a late 

 neurula. The organizing power of the heart 

 region is unable to overcome the determina- 

 tion present in most of the mesoderm after 

 gastrulation — at least, it is unable to induce 

 heart from presumptive somite mesoderm 

 from neurula stages (Bacon, '45). On the 

 other hand, mesoderm immediately adjacent 

 to the heart region is still competent to form 

 heart in tail-bud stage Amblystoma embryos, 

 as indicated in Figure 158. 



Is the presumptive heart mesoderm itself 



