446 



Special Vertebrate Organogenesis 



represented in each half rudiment whereas 

 in the former case a presumptive ventricle 

 must form atrium or vice versa. It has been 

 noted previously that the technique is more 

 difficult for combinations of posterior halves 

 than for lateral halves. In order to eliminate 

 the possibility that a transplanted half is 

 resorbed and superseded by host mesoderm, 

 the transplantations should be made hetero- 

 plastically on species with pigment differ- 



a normal asymmetry whereas the right one 

 frequently develops as a mirror image of 

 the normal. This is seen particularly well 

 in amphibians where experimentally pro- 

 duced double hearts undergo complete 

 fvmctional differentiation, with circulation 

 through each heart. Factors responsible for 

 the situs inversus of the right member of 

 a double heart are not clearly understood. 

 Further questions on cardiac asymmetry will 



First 

 Aortic Arch 



Ventricle 



Atrial Primordia 

 v • (unfused) 



(fused) 



Future Sinus Level 



Fig. 161. Progressive fusion of paired primordia during morphogenesis of chick heart. A, at 9-somite 

 (± 29 hour) stage, when first contractions appear; B, at 16-somite {± 38 hour) stage, when blood circula- 

 tion begins. (Redrawn after Patten and Kramer, '33.) 



ences in the embryonic tissues (e.g., Triturus 

 taeniatus and T. cristatus). 



Experiments by Goss ('35) show that some 

 totipotency exists in the primordium of the 

 mammalian heart. He found that double 

 hearts develop in 9-day-old rat embryos in 

 hanging drop cultures when the lateral 

 primordia are prevented from uniting. The 

 lateral rudiments self-differentiate beyond 

 the stage when they luiite in normal de- 

 velopment. Available experimental methods 

 do not maintain the growth of separated 

 mammalian heart primordia for sufficient 

 time to show whether a half can develop 

 a fully differentiated mammalian heart 

 with two atria and two ventricles. 



When double hearts develop from sepa- 

 rate bilateral primordia, the left heart has 



be considered in relation to asjonmetry of 

 entodermal derivatives. 



Formation of the "Tubular" Heart and 

 Functional Differentiation. Fusion of the bi- 

 lateral heart primordia occurs in a cephalo- 

 caudal sequence, with formation of the 

 bulboventricular region first and the sinus 

 venosus last. Space does not permit reference 

 to all of the studies which have established 

 this point for amphibians, birds and mam- 

 mals, including man (Davis, '27). Likewise, 

 it has been demonstrated that functional dif- 

 ferentiation progresses in a cephalocaudal 

 direction. Sabin ('20) and Johnstone ('25) 

 appear to have been the first to note that 

 the earliest contractions in the chick heart 

 begin in the ventricle. Using a cinemato- 

 graphic method, Patten and Kramer ('33) 



