HISTOGENESIS AND STEUCTUEE 99 



fractures occur at the Z lines. These membranes extend completely across 

 the fiber,, through the axial strand of sarcoplasm a significant fact con- 

 troverting the cellular idea of cardiac muscle originally advanced by 

 Apathy. The interstitial granules of Kolliker (sarcosomes of Eetzius) 

 scattered throughout the Q and J disks in 

 striped muscle, both cardiac and skeletal, 

 are designated the Q and J granules re- 

 spectively (Holmgren). 



INTERCALATED DISKS. A unique char- 

 acteristic of cardiac muscle pertains to the 

 presence of the intercalated disks, func- 

 tional lines/ or bands of Eberth. These 

 are barely visible in ordinary historic 

 preparations, but can be rendered conspic- HUMAN FETUS AT SEVEN 

 uous by the special technics of Heidenhain MONTHS. 

 and of Zimmermann. In gross appear- Fibrillae are well developed at 

 . , , the periphery: the undifferenti- 



ance they are of several sorts: straight ate d cytoplasm in the center 

 bands, step-like forms, and serrated forms, presents a clear appearance and 



The bands (disks) may extend completely in some cases is Partially occu- 

 ni -in o -i pied by the nucleus. Hematem 



across a fiber, or only the width of a single J^nd e0 g in> x 75a 



fibril (granule type) ; the step form may 



consist of one or many groups of steps and risers, the 'risers' being the 

 height of one or, occasionally, several inokommata; the saw-tooth type 

 also may be of small or greater extent, and of the height of one or several 

 inokommata. All three types may be arranged in rings or even longer or 

 shorter spirals. The intercalated disks are peripheral in position, extend- 

 ing for varying depths, but never completely through a fiber, and never 

 central to the axial sarcoplasm. They are occasionally on the same level 

 with the nucleus. They have been found in the heart of representatives 

 of all the animal groups to', and including, teleost fishes (Jordan and 

 Steele). They are present sparsely and in simple form also in the heart 

 of Limulus. They are probably a morphologic incident of the rhythmic 

 contraction of cardiac muscle. They appear only late in fetal life, toward 

 the end of the last week of gestation in the guinea pig. 



The earliest disks are all of the coarsely granular band type. Subse- 

 quently they increase in number and complexity, the older stages being 

 characterized by occasional saw-tooth forms. Once formed, they are 

 evidently for the most part permanent structures, undergoing modifica- 

 tion largely through mechanical factors. On closer inspection, under 

 the higher powers of the" inicrescope, they 'are seen to consist of units 



