INTERCALATED DISCS OF HEART MUSCLE 



163 



ficially and are of little depth, and apparently promiscuously 

 placed. On close examination, under higher powers, the appar- 

 ently compact delicate discs are seen to be granular. They are 

 abundant only in certain areas, and not generally as plentiful 

 as this particular illustration would indicate. 



Fig. 16 Four adjacent fibers of turtle heart. The discs are superficial, numer- 

 ous, and very like those of bird heart; always at anisotropic levels, usually com- 

 pact and only very rarely in 'steps.' 



Fig. 17 Cardiac muscle of toad. 



9. Frog 



Practically the same description holds for frog (fig. 18) as for 

 toad. The important point is the presence of discs in tissue 

 below birds, where they have been denied. There appears abso- 

 lutely no evidence here that they mark cell boundaries. 



