316 H. E. JORDAN AND J. B. BANKS 



conditions, probably excessive stresses, causing an inability on 

 the part of contraction bands to revert to the relaxed condition, 

 and as such subsequently chemically and mechanically modified, 

 can embrace more of the actual observational data and come 

 nearer expressing the real significance of the discs than any 

 hypothesis hitherto proposed. 



In support of this hypothesis numerous comparative develop- 

 mental and structural data must be considered. In the first 

 place, the gi'osser developmental features and alterations must 

 be kept in mind and brought into line. In the interpretation 

 of the intercalated discs not enough attention has hitherto been 

 given to the gross changes in the trabeculae: (1) The myocar- 

 dium is originally and definitely a syncytium, both with respect 

 to the grosser anastomoses between the trabeculae through their 

 branches and with respept to the myofibrils; however, towards 

 the end of the second fetal month, the myocardium consists of 

 closely compacted slender fusiform elements, resembling adult 

 conditions in smooth muscle, but the delicate intercellular 

 bridges with the continuous myofibrils meanwhile still effect a 

 syncytial structure. (2) The more delicate trabeculae of the 

 later fetal myocardium lengthen and coarsen through fusion 

 and intrinsic growth, and the originally more delicate branches 

 undergo similar changes and in many instances alter their origin 

 at sharp angles to origins at less acute angles. (3) In the proc- 

 esses of later development the ventricular fiber-bundles and 

 their constituent trabeculae undergo spiral twistings (e.g., the 

 bulbo-spiral band: Mall (18), Am. Jour. Anat., vol. 11, 1911, 

 fig. 19, p. 262) which involve secondary fusions of adjacent 

 trabeculae and distortions of branches, including in certain 

 cases an inturning of a portion of the originally peripheral sarco- 

 lemma (see also Heidenhain's "Plasma und Zelle," figs. 297e, 

 298 and 300, and diagram fig. 353, p. 616; and Jordan's (12), 

 "Studies in Striped Muscle Structure," No. Ill, Anat. Rec, vol. 

 6, 1917).. 



With respect to the finer microscopic features, the following 

 changes must be kept in mind: (1) The discs are invariably re- 

 lated spatially to the telophragmata, being bounded on one or 



