PLAT1<: 3 



EXPLANATION OF FIGURES 



34 Portion of a longitudinal section of a trabecula from the moderator band, 

 showing the union of the telophragmata to the nuclear wall and to the festooned 

 sarcolemma. No indication appears of an additional mesophragma. Iron- 

 heniatoxylin and van Gieson's stains. 



35 Longitudinal section of a portion of the cellular network from the right 

 limb of the atrioventricular bundle. The majority of the cells contain two 

 nuclei very closely associated. Occasional cells contain three or four nuclei. 

 The cells are polyhedral in shape, variously modified (figs. 37, 42, 43 and 44). 

 The margins appear serrated in sections (in macerated preparations the cell 

 meml)rane has a sharp contour; fig. 44). The serration in histologic prepara- 

 tions is an artifact due to the non-uniform shrinkage between the myofibrils 

 and the sarcolemma, to both of which the telophragmata are attached. The 

 sarcolemma stains more deeply, as do also the lateral attached portions of the 

 telophragmata. X 108. 



36 More highl}- magnified portion (.r) from figure 35. The intracellular myo- 

 fibrils are seen to be aggregated into smaller groups, more abundant peripherally. 

 The peripheral groups follow in general the cell contour; hence the myofibril- 

 groups are irregularly disposed with respect to each other, and the telophragmata 

 are distorted and apparently in places interrupted. Numerous intercellular 

 spaces occur. Certain fibrils of the myofibril-bundles are continuous from cell 

 to cell, forming thus a syncytium in spite of distinct cell-walls. The sarco- 

 lemma and the attached portions of the telophragmata stain more deeply than 

 the central fibrils. X 666. 



37 More highly magnified cell, of modified polyhedral form, from the portion 

 of the atrioventricular bundle shown in figure 35. 



38 Small area of ventricular myocardium of fetal heart towards end of sec- 

 ond month, showing several adjacent muscle cells in longitudinal section. The 

 cells are long, slender, fusiform elements resembling definitive smooth-muscle 

 cells. The myofibrils are apparently continuous from cell to cell. They are 

 relatively meagre in amount and peripherally arranged. The telophragmata 

 are conspicuous but very delicate. Peripherally, among the fibrils, Q-discs 

 are also barely discernible midway between successive telophragmata. Simple, 

 delicate, deep-staining, granular intercalated discs also occur peripherally at 

 telophragma levels, apparently as modifications of this membrane. The discs 

 do not occur in the areas of fusion between adjacent cells, but at right angles to 

 such surfaces. X 1000. 



39 Portion of longitudinal section of ventricular myocardium of fetal heart 

 of fourth month; showing the process of fusion of the slender fusiform cells to 

 form trabeculae, and a few simple band-like discs. 



40 Portion of longitudinal section of ventricular myocardium of fetal heart 

 of seventh month, showing numerous band-like discs. 



41 Portion of a longitudinal section of a coarser trabecula from an adult 

 Limulus-heart, including two intercalated discs, apparently forming a two-step 

 type, separating an upper contracted from a lower uncontracted region. The 

 discs are at telophragma levels and apparently represent modified contraction 

 bands. The illustration shows clearly the close resemblance between the discs 

 of the adult Limulus heart and the simplest type of disc of mammalian lioarts. 



{Continued on page 338) 

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