INTERCALATED DISCS OF THE HEART OF BEEF 311 



a two-month heart for study of the origin of the cells of the 

 atrioventricular bundle, and the manner by which they became 

 bi nucleated. This tissue was deeply stained and at once clearly 

 showed intercalated discs in the fusiform cells. The resemblance 

 between the histologic features of the heart of the beef-fetus 

 at two months and those of the adult toad-heart, for example 

 (Jordan and Steele), is striking. Tissue from the four-month 

 heart was then restained, when the discs became clearly visible. 

 And in the seven-month heart the discs were abundant and of 

 substantially identical structure and relationship, except as 

 altered by a coarsening, and the extraneous mechanical factors 

 incidental to development, which factors continue to operate as 

 modifying influences through postnatal growth and development. 



As concerns the myofibrillar elements the early fetal heart is 

 syncytial. But at two months, fusiform cells are plainly dis- 

 tinguishable. They contain an oval vesicular central nucleus. 

 Certain nuclei are in process of amitotic division. The myo- 

 fibrils are . sparse and peripherally arranged. The telophrag- 

 mata are delicate but conspicuous, and peripherally among the 

 fibrils Q-discs are faintly discernible. Delicate, deep-staining, 

 granular discs appear fairly abundantly peripherally, apparently 

 as modifications of the telophragmata (fig. 38). The cells are 

 beginning to fuse to form the coarser trabeculae characteristic 

 of later developmental conditions. It seems probable that the 

 roughly-dichotonous division of the trabeculae of succeeding 

 earlier stages, characteristic also of the moderator band (fig. 29), 

 results from a central fusion of such fusiform cells, the branches 

 representing the more widely spaced and unfused distal pointed 

 ends of these fusiform elements. These terminals fuse with 

 other similar terminals in the formation of the more regular 

 meshwork of the later fetal heart. 



The discs appear at right angles (approximately) to the sur- 

 face of fusion, rarely in the surface of fusion as when two cells 

 fuse end to end. Where cells fuse in this manner, along oblique 

 surfaces, a recoordination of the constituent myofibrils must be 

 effected along such surfaces, and the stresses involved may effect 

 the modification of the myofibrils which constitute the inter- 



