444 



Special Vertebrate Organogenesis 



ciirvature can be influenced by the shape of 

 the pericardial cavity and by other mechani- 

 cal factors. 



In birds, the cardiac primordia have 

 greater potentialities for histological than 

 for morphological self-differentiation. Small 

 pieces of chick blastoderm will develop typi- 

 cal cardiac muscle with rhythmical pulsa- 

 tions in culture (Olivo, '28), but they do not 



amount of morphological self-differentiation 

 for the rat heart, but it must be noted that 

 neither in mammals nor in birds has morpho- 

 logical differentiation been demonstrated for 

 cardiac primordia completely isolated from 

 other embryonic tissues. 



An abundance of evidence could be cited 

 to show that striicture and function are not 

 dependent upon each other during the early 



23 Hrs. 



50 Hrs. 



Fig. 160. Self-differentiation of presumptive heart material cultured in modified Holtfreter's solution. 

 A, Amblystoma punctatum, stage 11. Explant of gastrula wall from outlined area lateral to the blastopore 

 differentiates as shown in B. Arrows show direction of contraction wave at 84 hours after explantation. 

 C, differentiation of presumptive mesoderm explanted from an embryo of stage 15, late medullary plate 

 stage. (Redrawn after Bacon, '45.) 



show morphologically differentiated hearts 

 comparable to those formed by amphibian 

 explants. Chorioallantoic grafting of heart- 

 forming areas of the chick blastoderm gives 

 more cardiac morphogenesis than in vitro 

 cultivation does, but morphogenesis is still 

 inferior to histogenesis (Kume, '35; Rawles, 

 '43). Rawles suggests that the atypical 

 morphogenesis may be the result of abnormal 

 mechanical conditions rather than a restric- 

 tion of the determination process. 



In mammals, as in birds, the tendency for 

 cardiac self-differentiation is greater physio- 

 logically than morphologically, as shown by 

 the behavior of whole blastocysts cultivated 

 in vitro (Waddington and Waterman, '33; 

 Nicholas and Rudnick, '34; Goss, '35). The 

 studies by Goss have shown a considerable 



development of the heart. Bacon ('45) finds 

 explanted amphibian hearts may differen- 

 tiate morphologically without developing 

 pulsations, whereas contractions may occur 

 in explanted rudiments lacking morphologi- 

 cal differentiation. Numerous studies al- 

 ready cited on self-differentiation have 

 shown that a lack of blood circulation does 

 not prevent the heart from developing well 

 beyond the stage which it normally attains 

 at the beginning of circulation. On the other 

 hand, there is evidence that the blood stream 

 has an influence on the later stages of heart 

 differentiation in both amphibians and birds 

 (Stohr, '25; Bremer, '31). In the chick heart, 

 a valvelike action occurs at the a-v junction 

 through an "endocardial mound" of cardiac 

 jelly before the establishment of the pri- 



