( 224 ) 



as minute black grannies (dii)losonics) in the centre olan ovalshaped 

 -heller Hof'. 



Thej were l.ying on the side of the nnclens in a rather indifferent 

 l)Osition now on this side of the cell, than on that. 



In the course of development the cells of the myocardium flatten 

 more and more. The cell membranes disappear from between the 

 cell bodies throughout the entire thickness of the myocard. In this 

 stage the fibrillae are not so exclusively confined to the l)asal side 

 of the cells, but are found more or less scattered throughout the 

 cells. The greater part of the fibrillae, however, is still visible in the 

 basal half of the heart muscle syncytium. 



In fig. 5 a surface view (of the atrium) of such a myocard is 

 drawn. Beneath the fibrillae Ave see three nuclei, no trace whateve)* 

 of cell boundaries is visible. The course of the fibrillae is not so 

 regular as it was before. They seem to be running now in (liirc]-ent 

 dii-ections, although there is still one predominant course. This fact 

 is due to the rate of growth of the heart tube being not the same 

 in different directions. The heart has no more the shape of a simple 

 cylindrical tube, but is differentiated already in sinus venosus, atrium 

 and ventricle. With the growth of the different parts of the heart 

 tube the dis|»lacing of the bundles of fibrillae goes hand in hand. 



As shown in the figure, the myofibrillae of the heart now i)resent 

 a beautiful cross striation. Hut it must be noticed that the commen- 

 cement of the functions of the heart muscle, of rhythmical i)eristaltic 

 contractions coincides with the dilferentiation of the homogeneous 

 fibrillae mentioned above. The diflercntiation of the fibi-illae in 

 isotropous and anisotro[)Ous discs takes [)lace after the heart having 

 contracted (piite regulai-ly for a long time already, and has nothing 

 to do with the contractility of the fibrillae. 



In this «tage of deAeloi)ment (the last stage which can l)e studied 

 here, the muraenoid larvae invariably dying after having reached 

 the critical period) the wall of the heart is still a simple membrane. 

 The bundles of muscle fibres so characteristic for the adult vertebrate 

 lieart are not yet developed. For this reason I have reproduced in 

 fig. 6 a part of a longitudinal section of the myocard of a lar\a 

 of Salmo fario of 22 m.M., where the sponge-like structure of the 

 myocard was established already. The myofibrillae, for the greater 

 part arranged in bundles, may be folio vA'ed o\'er a great area past 

 difïerent nuclei of the myocard-syncytium. There is no trace of 

 cellular limits, nor of " Schal tstücke" te be found. 



So we must draw the conclusion, that in the case of leleosts too 

 the mvocard forms a svncvtium, as maintained by Godlkwski, 



