Heart, Blood Vessels, Blood, and Entodermal Derivatives 



443 



dependent upon a secondary entodermal in- 

 ductor for its differentiation? Nieuwkoop 

 ('46) found an absence of the heart in Triton 

 embryos which developed after complete 

 removal of the entoderm from early neurula. 

 Bacon (personal communication) has ob- 

 tained similar results for Amblystoma oper- 

 ated upon at Harrison's stages 15-18, and 

 Balinsky ('39) has reported similar findings 

 for nevirula stage Tritons. One finds it diffi- 

 cult to explain the lack of heart development 

 in entodermless embryos in view of other 

 evidence favoring ability of very early stages 

 for cardiac self-differentiation. 



Heart Determination and Self-Differentia- 

 tion. The first experimental evidence indi- 

 cating the stage from which the presumptive 

 heart-field can self-differentiate into an or- 

 ganized heart was obtained by Ekman ('21). 

 Using Bombinator embryos at the stage when 

 the heart is first visibly indicated, he ex- 

 planted the heart primordivim in an ecto- 

 dermal covering. Using similar methods 

 and materials, Stohr ('24a) concluded that 

 the heart differentiation in the explants 

 was correlated with the presence of neigh- 

 boring entodermal and mesodermal cells re- 

 moved with the heart. Further evidence on 

 the problem has been obtained from both 

 explantations (Ekman, '24, '27, '29; Bacon, 

 '45) and heterotopic transplantations (Stohr, 

 '24b; Copenhaver, '26). The former method 

 permits a study of heart formation free of 

 confinement and association with other tis- 

 sues; the latter method permits a study of 

 the organ freed only from the structures 

 with which it is normally associated but it 

 gives more differentiation than the isolation 

 method and enables one to identify the dif- 

 ferent heart chambers with more assurance. 

 Stohr's results with this method reempha- 

 sized his belief in the importance of "Endo- 

 mesodermzellen" which were always present 

 in successful transplants. On the other hand, 

 heterotopic transplants on tail-bud stage 

 Amblystoma embryos (Copenhaver, '26) 

 showed that well formed pulsating hearts 

 can develop from presumptive heart meso- 

 derm without the normally associated ento- 

 mesodermal structures. Particularly strong 

 evidence for the self-differentiating capacity 

 of the heart has been obtained with the ex- 

 plantation method by Bacon ('45), as shown 

 in Figure 160. 



The explantation method has been em- 

 ployed in studying the time of heart deter- 

 mination in both anurans (Ekman, '27, '29) 

 and urodeles (Bacon, '45). Bacon's results 

 indicate that the presumptive heart material 



of Amblystoma is determined and capable 

 of self-differentiation into typical parts in 

 the crescent blastopore stage (stage 11). But 

 the explants at this stage consisted of a 

 piece of gastrula wall which may have in- 

 cluded neighboring entomesodermal cells. 

 The evidence for self-differentiation is un- 

 equivocal in the experiments done on medul- 

 lary plate stages (stages 13-15) since only 

 mesoderm was explanted. Thus the heart 

 primordium is self-differentiating for a con- 



0.7 mm. 



Fig. 159. Map of heart-specific areas on a head- 

 process stage chick blastoderm. Numerals at right 

 show distances in millimeters from the level of the 

 primitive pit. (After Rawles, '43.) 



siderable time before the environment of 

 the cardiac region loses its ability to induce 

 heart formation in indifferent mesoderm, 

 as cited earlier. 



The heart primordium appears to have a 

 "labile determination" for curvature at an 

 early stage. Some studies have suggested 

 that the heart possesses merely a tendency 

 for curvature since the particular form ob- 

 served under experimental conditions is vari- 

 able and obviously influenced by the en- 

 vironment (Spemann, '06; Pressler, '11; Ek- 

 man, '24; Stohr, '24a, '25; Copenhaver, '26). 

 But the results obtained by Bacon ('45) for 

 hearts explanted without confinement show 

 a determination for the characteristic S-type 

 of curvature. This does not deny the view 

 that the S-shape is suited to the environ- 

 ment in which the heart normally develops 

 nor does it contradict other evidence that the 



