Heart, Blood Vessels, Blood, and Entodermal Derivatives 



447 



have extended these studies to give us a 

 detailed account of the series of changes 

 by which the sinus venosus is eventually- 

 established as the pacemaker. An excellent 

 review of this work has been presented by 



birds, mammals exhibit less fusion and am- 

 phibians more. In urodele amphibians, the 

 so-called "tubular" heart of the beat initia- 

 tion stage contains the future conus, ven- 

 tricle and cephalic portion of the atrium 



Globules 



Nuclei 



Fig. 162. Heart differentiation in Amblystoma punctatum at stage 34, when first contractions appear. 

 A, heart in situ after removal of ectoderm and ventral portion of parietal pericardiimi. B, enlargement of a 

 portion of A. The outlined area on the bulboventricular region shows extent of contractions observed before 

 fixation. X and Y indicate levels of sections illustrated in C and D, respectively. Note abundance of yolk 

 droplets and lack of striations in myocardium. 



Patten ('49). Figure 161 shows some of the 

 morphological stages in the cephalocaudal 

 differentiation. 



Studies on the genesis of cardiac contrac- 

 tions in mammals (Goss, '38; Dwinnel, '39) 

 and in amphibians (Copenhaver, '39a) have 

 given results which differ in only a few 

 respects from those described for the chick. 

 The degree of fusion of the bilateral pri- 

 mordia at the time of beat initiation varies 

 for different animals; in comparison with 



(Fig. 162B). At this time, only the cephalic 

 portion of the heart is invested completely 

 by primordial myocardium. When dealing 

 with amphibian development, some embry- 

 ologists retain the mistaken impression that 

 a "tubular" heart of four regions is formed 

 all at once. It can be seen that this view 

 is incorrect when Nile blue-stained areas 

 of the early heart are identified in later de- 

 velopment (Copenhaver, '39a). 



Although the initiation of cardiac fiinction 



