172 H. E. JORDAN AND K. B. STEELE 



V. SUMMARY AND CONCLUSIONS 



1. By the use of Zimmermann's technic it was possible to dem- 

 onstrate intercalated discs in all heart muscle examined, except 

 that of Limulus. Of lower vertebrates the material included that 

 of turtle, lizard, frog, toad and trout, in which forms the presence 

 of discs has been denied. 



2. In guinea-pig, in which form only the matter was tested, 

 intercalated discs appear early during the last week of gestation, 

 coincidently with the appearance of striations. A progressive 

 increase in number, complexity and density was noted during 

 the first, second, third and fourth weeks of post-natal life. A 

 similar more pronounced difference obtains between the heart of 

 the young and that of the adult guinea-pig. In a cat embryo of 

 four days the discs are already present but few in number, pale, 

 and loosely granular in structure. 



3. Compared with mammals (e.g., monkey, bat, chipmunk), 

 in lower vertebrates the discs become progressively less numerous 

 (except in birds, e.g., humming-bird), narrower, less compact 

 (more granular) and less complex. Conditions with respect to 

 the discs in young mammalian hearts are very similar to those in 

 the hearts of lower vertebrates. With increase of age, there is a 

 progressive increase in number, complexity, density and width 

 of discs (e.g., guinea-pig). 



4. The comparative study of vertebrate heart muscle gives no 

 evidence favoring the interpretation of the discs as structures 

 marking cell boundaries, e.g., cement lines or intercellular bridges. 



5. Specific points in the evidence against an intercellular inter- 

 pretation are: (a) their superficial location; (b) their relationship 

 to the dark ('anisotropic') band, i.e., they displace these bands and 

 shade laterally into them; (c) their position frequently over a 

 nucleus; (d) their relation to the myofibrillae; (e) their random 

 arrangement with respect to the nuclei; (f) structurally and 

 tinctorially they seem to be of the same nature as the so-called 

 anisotropic substance; and (g) their absence before the appear- 

 ance of striations. 



