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



5. Opossum 



Conditions in the heart of the opossum are almost identical 

 with those in guinea-pig. As concerns the intercalated discs, 

 they differ only from their homologues in guinea-pig in being 

 somewhat paler, more delicate and apparently less numerous. 

 The deeper-staining character of the disc-containing portion of 

 the muscle trabeculae is especially conspicuous in the opossum 

 heart. 



6. Humming-bird 



Fig. 15, added chiefly for the purpose of completing the series 

 of illustrations, shows a fiber of humming-bird heart muscle to 

 illustrate the relative abundance, comparative form, and typical 

 appearance of this tissue. Humming-bird heart muscle has been 

 fully described in a former paper 7 by one of the authors. 



7. Turtle 



In turtle heart the discs are fairly abundant (fig. 16). The 

 discs here all appear as narrower or wider plates. They are again 

 superficial and at the anisotropic levels, displacing the darker 

 bands, and in many instances shading gradually into them at one 

 or both ends. None of the rare varieties above described for 

 mammalian muscle appear in turtle nor in lower forms, even 

 step-like discs being very rare. The localization of the discs in 

 definite transverse areas is apparently absent. The discs here 

 are stouter than in frog and toad. 



8. Toad 



Fig. 17 illustrates conditions in the heart muscle of toad. 

 The majority of discs are very narrow, though occasionally discs 

 the width of an entire fiber appear. The discs are situated super- 



7 In this paper the darker band was regarded as the Q band without con- 

 sideration of the possibility that it may represent the Z line modified in con- 

 traction. This possibility is discussed below. 



