THE PHY81 \ ■ 171 



sidered as absolutel) conclusive AlthouRl 



at firsl sighl appear t<> be conclusiv< . • • 



subjected to a closer scrutiny It is o 



evidence for and against each view thai we shall In- in a 



to any conclusion, and • then it will be plain thai 



be only tentath • 



Myogenic Hypothesis 



Taking firsl of all the evidence in Buppoii of the my< g 

 the following Btanda oul mosl prominently: 



1 The heaii beats in the embryo chick b< any n< 

 grown into it. and no1 only this, bul if porl 



moved from the embryo and placed in l»l 1 plasma, ontinue 



ting for many days It has also been observed tl a1 may 



off from this mass of cardiac muscle and undergo multi] 

 differentiation, so a^ to produce isolated musch a which exhibit 



rhythmic contractility. The rebuttal on the part 

 ihis apparently unassailable « - x i « that, i 



bryonic muscle cells may exhibil the power of rhythmic i 



does nut mean thai the fully developed muscle i arily 1 



such power. In the eary stag inic development, il 



evidenl thai the functions which in the fully developed animal . 

 to various Bpecial organs and ti should be performed b 



having Beveral such functions in common. The muc 

 for example, maj to Btarl with be power no1 



tracting bul also of initiating the contraction. It maj 

 tly nervous in character and that only later, when the di 

 mmmated, does the power of rhythmic contractioi 

 gated to the nervous element and tl.. 

 muscle itself. 



2 The aervous structure in the I aaj be damaged - 

 chanical means or by drugs withoul apparently int< with 

 power of rhythmic contract i 



it is possible to d ml a c< nsid< i amoui I 



withoul a: the beat, and in all auin 



of atropine, w hich pai the p 



nervous bj stem m •• page 22< ml in I 



tricle in such h< 



I n. I>\ 



and although a few n< 

 withoul virtue 



made to contract rhythmically !•> p< 



