INTERCALATED DISCS OF THE HEART OF BEEF 317 



both sides or bisected by them, and, at least occasionally, shad- 

 ing laterally into these membranes. (2) The telophragmata are 

 in close union with the myofibrils, the sarcolemma, and the 

 nuclear wall. (3) The discs are peripherally placed and consist 

 of associated local modifications of adjacent myofibrils. 



The developmental features that require emphasis in this con- 

 nection are: (1) The absence of discs in the embryonic myocar- 

 dium; the discs appear only gradually in fetal life (beginning 

 about the second month) as delicate peripheral bands, apparently 

 as thickenings of parts of the telophragmata; they increase in 

 number, size and variety during the period of the growth of the 

 heart; these developmental changes recapitulate the phylo- 

 genetic history of the discs, as first pointed out by Jordan and 

 Steele (14), who found them in hearts from teleost fishes to 

 birds, and even in the Limulus heart (Jordan (9 and 11) ) where 

 they are exclusively of the simple-comb type (sometimes in the 

 shape of a two-step form), located at telophragmata levels. (2) 

 The more complex tj^es of discs can all be refelred to the 

 simpler band types, as mechanical secondary modifications of 

 these simpler types. (3) The simplest discs consist of rows of 

 bacillary modified foci on adjacent fibrils., (4) Hypertrophied 

 and atrophied pathologic myocardia are characterized by definite 

 types of discs, complex serrated forms and narrow comb forms 

 respectively (Dietrich (3); Jordan (6, 7 and 13) ). 

 • The close structural similarity of the original and simplest discs 

 to contraction bands, and their identical location with respect to 

 the telophragmata, suggested an origin of discs from modified 

 contraction bands. A contraction band in a stained section of 

 certain insect muscle fibers (leg or wing; Jordan (10) ) has the 

 appearance of the simplest type of intercalated disc. If it be 

 assumed that certain bands, on account of excessive strains, 

 become incapable of reversion, then the possible beginnings of 

 discs seem to be present, which simple discs are correctly con- 

 ceived to be capable of modification through the operation of 

 mechanical factors into the various types of discs above described. 



It seems desirable at this point to trace the probable steps, as 

 suggested in the histologic preparations, by which the more 



