458 



Special Vertebrate Organogenesis 



are capable of self-differentiation but ex- 

 plants from the left side give a higher per- 

 centage of positive cases and an earlier initia- 

 tion of pulsations. Rawles ('43) obtained 

 essentially similar results for chorioallantoic 

 grafts of heart-forming areas from the early 

 chick blastoderm. 



Situs inversus of the heart and gut fre- 

 quently occur together although either one 

 may appear without the other. This is true 

 both for the cases seen in nature and for 

 those produced experimentally. The most 

 commonly described situs inversus cordis in 

 the absence of situs inversus viscerum is that 

 which occurs in the right member of double 

 hearts produced by prevention of union of 

 the bilateral heart primordia. In these cases, 

 left hearts have normal asymmetry; right 

 hearts frequently show situs inversus. The 

 asymmetry of the right member of double 

 hearts is probably correlated with an en- 

 vironmental influence already discussed by 

 Ekman ('24, '25) — the union of the heart 

 with the blood vessels and the pressure ef- 

 fects within the pericardial cavity favor 

 normal curvature on the left side and situs 

 inversus on the right side when the bilateral 

 heart primordia develop separately. It is not 

 surprising that the right member of double 

 hearts does not develop situs inversus in- 

 variably, that both hearts occasionallv show 

 the same asymmetry (Fales, '46). Without 

 the environmental effects jvxst cited, normal 

 asymmetry of the right member of double 

 hearts might occur more often than it does. 

 Theoretically, one might expect right heart 

 primordia freed from the influence of the 

 left side to develop with normal asymmetry 

 and with situs inversus in about eoual num- 

 bers, similar to the results obtained by Ruud 

 and Spemann ('23) for right half blastulae. 

 They found that when blastulae are com- 

 pletely constricted, left halves develop into 

 small animals with normal asymmetry while 

 right halves develop with situs inversus and 

 with normal asymmetry in about equal 

 numbers. For further details on the relatio^^ 

 of situs inversus to localized defects in the 

 germ layers and the probable relationship 

 between normal visceral asymmetry anr^ 

 asymmetry in the embryonic "organizer," 

 reference should be made to von Woellwarth 

 ('50). 



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