448 



Special Vertebrate Organogenesis 



in the different classes of vertebrates appears 

 to be timed appropriately to the needs of 

 the embryo, it must be emphasized that the 

 initial contractions are not dependent upon 

 conditions within the embryo or upon the 

 development of heart form. This has been 

 shown by explantation, transplantation and 

 tissue culture methods. Furthermore, the 

 initial contractions do not appear to be 

 dependent upon the presence of striations. 

 Goss ('38, '40, '42) has observed cardiac 

 contractions in rat embryos before striations 

 can be seen in the myocardial cells. Simi- 

 lar conclusions have been given for Am- 

 blystoma (Copenhaver, '39a). In the chick 

 heart, contractions have been observed at the 

 9-somite stage (Patten and Kramer, '33), 

 whereas striations have not been observed be- 

 fore the 10 somite stage (Lewis, '19). In this 

 connection, it should be noted that studies 

 with the electron microscope (Schmitt, '45) 

 have shown that the absence of histological 

 signs of striations need not imply the ab- 

 sence of molecular or micellar striations. 



In the preceding discvission, reference has 

 been made to the fact that different parts 

 of the embryonic heart have inherently dif- 

 ferent rhythms. As early as 1890, Fano and 

 Badano cut chick embryo hearts with refer- 

 ence to definite anatomical levels and re- 

 corded higher pulsation rates for the atrium 

 than for the ventricle. These early observa- 

 tions have been amply confirmed and ex- 

 tended by later workers. Similar findings 

 have been made for embryonic mammalian 

 hearts (Hall, '51). That the part with the 

 highest rate dominates the other regions 

 was demonstrated clearly by Paff ('36). He 

 grew reversed parts of the chick heart in 

 proximity to each other in culture media 

 and observed the sinoatrial region imposing 

 its rate on the slower beating anterior part 

 when the two pieces became united by a 

 bridge of myocardial tissue. Further evi- 

 dence for sinus dominance is seen when 

 parts of the embryonic heart are trans- 

 planted between different species of am- 

 phibians — a transplanted sinus venosus 

 dominates the remainder of the heart of 

 another species to the point of maintaining 

 the donor species rate (Copenhaver, '45). 



Figures 163 and 164 summarize quanti- 

 tative studies on the intrinsic rhythms of 

 each of the cardiac regions at successive 

 developmental stages in Amblystoma piinc- 

 tatum (Copenhaver, '39a) and in the chick 

 (Barry, '42). The rhythm of each part passes 

 through a phase of rate acceleration which 

 is generally followed by a period of decelera- 



tion. Eventually, some of the parts enter a 

 phase in which they lack the ability for 

 spontaneous contraction. The conus of the 

 chick heart enters the latter phase very 

 early and its rate is omitted from the figure; 

 the conus of the Amblystoma heart exhibits 

 spontaneous contractions intermittently for 

 a much longer time than in the chick. An- 

 other difference between the two species is 

 found in the behavior of the sinus venosus 

 which enters the deceleration phase earlier 

 in Amblystoma. In the chick, there is a 

 question whether the rhythm of the intact 

 heart (and the rate of its sinus) shows any 

 deceleration phase. At most, there is only 

 a slight decrease in rate after abovit 17 

 days of incubation (Cohn and Wile, '25) 

 or merely a plateau in the rate curve at 

 that time (Bogue, '33). 



Numerous studies have shown that the 

 embryonic heart rate exhibits a progressive 

 acceleration which is most pronounced in 

 the early stages of cardiac function. This 

 appears to be true in fish, amphibians, birds 

 and mammals. Among the studies on chick 

 heart rates, those bv Cohn and Wile ('25), 

 Bogue ('32) and Barry ('40) are particu- 

 larly significant. In both the chick (Fig. 

 164) and Amblystoma (Fig. 163), one notes 

 a particularly obvious acceleration in heart 

 rate just after the establishment of atrial 

 function, and again just after the initiation 

 of sinus function. The available evidence 

 supports the statement by Barry ('42) that 

 the progressive acceleration in heart rate 

 in the early stages of cardiac function is 

 "due in great measure to the successive 

 addition of new segments of myocardium of 

 increasingly higher inherent automaticity." 

 The progressive acceleration in the chick 

 heart rate in later stages has been correlated 

 with (1) an increase in blood presstire 

 (Barry, '41), or (2) a reduction in cardiac 

 distention following the deflection of blood 

 into newly formed arteries (Alexander and 

 Glaser, '41.) These contradictory views are 

 based on contradictory results obtained with 

 exsanguination experiments on chick em- 

 bryos — bleeding decreases the heart rate 

 according to Barry, and increases it ac- 

 cording to Alexander and Glaser. It seems 

 that the effects of hemorrhage on embryonic 

 heart rate could be profitably reexamined. 



The most significant studies correlating 

 stages of heart development and changes in 

 the electrocardiogram have been made on 

 chick hearts by Hoff, Kramer, DuBois and 

 Patten ('39). In the earliest electrocardio- 

 gram that can be obtained, there is a sinu- 



