Heart, Blood Vessels, Blood, and Entodermal Derivatives 



449 



soidal pattern produced by a simple de- 

 flection of the galvanometer. A deflection 

 resembling the QRS ventricular complex is 

 not obtained until about 38 hours incubation. 

 At this time, the contraction is still pri- 

 marily ventricular and only the cephalic 



ward at first; later it becomes reversed. An 

 approximately adult type of electrocardio- 

 gram is established during the fourth day 

 of incvibation. 



Nervous Control of the Embryonic Heart. 

 Since the work of His and Romberg (1890), 



3-1 35 36 37 38 39 40 41 42 43 44 

 STAGES OF DEVELOPMENT 



J I I L 



70 8C 



IN HOURS 



60 



AGE 



Fig. 164. 

 Figs. 163 and 164. Inherent rhythms of different regions of transected hearts in Amblystoma (Fig. 163) 

 and in the chick (Fig. 164). The stages shown for Amblystoma cover a 12-day period extending from the 

 time of beat initiation until the embryos begin to feed. S, A, V, and C, show rates for sinus, atrium, ventricle 

 and conus, respectively. Each point on the Amblystoma curves represents a mean for all cases studied, and 

 the difference shown for the sinus and the intact, normal heart (N.H.R.) is statistically insignificant. (Fig. 

 163 after Copenhaver, '39; Fig. 164 after Barry, '42.) 



portion of the atrium is functioning. Ac- 

 cording to Olivo et al. ('46), the establish- 

 ment of a QRS pattern is correlated with 

 the differentiation of myofibrillae and may 

 occur at a somewhat earlier stage than that 

 just cited. The P wave (atrial) appears 

 first at 42 hours incubation, soon after the 

 beat becomes established in the caudal part 

 of the atrium. The P deflection is down- 



it has been known that the embryonic 

 heart functions preceding innervation. This 

 indicates that the first heart beats are myo- 

 genic in origin. Furthermore, Hooker ('11) 

 observed rhythmical cardiac contractions in 

 denervated amphibian embryos reared be- 

 yond the stage when the heart of normal 

 embryos becomes innervated. Fvxrther evi- 

 dence for a myogenic origin of heart beat 



