INTERCALATED DISCS OF THE HEART OF BEEF 309 



post-natal to the adult conditions. But in the actual investi- 

 gation it was found necessary to pass in the reverse order, for 

 only in this way were these earliest fetal conditions correctly 

 interpreted. Conditions in the Purkinje fibers of the adult 

 heart served as the connecting link, and the interpretative key. 

 Once the intercalated discs were discovered and interpreted 

 in the Purkinje fibers (fusing cells) and in the early fetal heart, 

 these simple conditions threw much light upon their definitive 

 structure and relationships. Fetal and adult conditions served 

 mutually to disclose the correct interpretation of the discs. 



The youngest fetal heart studied was that of the end of the 

 second or the beginning of the third month. The ages specified 

 for the fetal hearts can only be regarded as close approximations. 

 The youngest heart measured 32 mm. from base to apex, and 25 

 mm. at its widest point. This heart takes us very close, if not 

 actually to, the first beginnings of the intercalated discs. At 

 this stage the ventricular myocardium consists of closely-com- 

 pacted, slender, fusiform elements (fig. 38). The resemblance 

 to smooth-muscle structure is striking. This resemblance has 

 not to our knowledge been previously pointed out. It is signifi- 

 cant from the point of \dew of comparative histogenesis that 

 cardiac muscle should pass through a transient phase of develop- 

 ment in which it resembles definitive smooth-muscle tissue. 

 Striped voluntary muscle of vertebrates likewise passes very early 

 in its histogenesis through a very similar condition. In their 

 earlier embryonic condition smooth muscle and cardiac muscle 

 both consist of stellate and irregular elements whose processes 

 have anastomosed to form a syncytium. The general idea that 

 smooth, cardiac and striped skeletal muscle represent essentially 

 successively higher stages of differentiation receives additional 

 support in the evidence that heart muscle and skeletal muscle 

 pass through a smooth-muscle stage. 



But the cardiac muscle even at this early stage contains 

 simple intercalated discs. The question then arises as to why 

 neither smooth nor skeletal muscle contain similar discs. The 

 answer probably inheres in the functional differences. The 

 rhythmic contraction of heart muscle even in early fetal conditions 



