( 225 ) 



Heidenhain, Hoykr and Marceau ; that the myoeard originally is formed 

 of distinct cells, hut that during the dilferentiation of the niyotibrillae 

 the cell limits of the myoeard cells get lost, the cell bodies fuse and 

 in this manner a syncytium is formed; that this disap[)eai"ance of 

 the cell membranes can be stated at tirst only thei-e where the 

 myofibrillae are formed, and that chronologically the formation of 

 the tibrillae and the disappearance of the membranes coincide. 



Where now the formation of this syncytinm by a fusion of 

 originally separated cells can be demonstrated in lower vertebi-ates 

 and in the higher vertebrates, and the continuity of the fibrillae over 

 a great area can be stated, there the hypothesis of Hkidknhain, lliat 

 in the adult mammalian heart the "Schaltstiicke" (cement lines) of 

 the myoeard which are not to be found in lower vertebrates and 

 which appear in mammals in a relatively late stage of de\'eIopment, 

 have nothing to do with cell limits, seems to have some trntli in 

 it. That this is of great importance for the phj^siology of the 

 heart muscle, for the problem of the conduction of the impulse 

 by the heart muscle fibres, I need only mention here. 



As to the functions of the "Schaltstiicke" we are, I think, still 

 entirely in the dark, but for the hypothesis of Heidenhain. This 

 must be tested by further study. The study of the later stages of 

 development of the mammalian heart with the use of the modern 

 histological methods will throw more light u[)on this, as was pointed 

 out already by Godlewski. Perhaps tlie study of tiie structure of 

 the muscular bridges, connecting the ditlercnt [)arts of the nuimmalian 

 myoeard, too will throw some light u}>on this question. 



DESCRIPTION OF THE FIGURES ON THE PLATE. 



Fig. 1. Longitudinal section througli the heart of an embryo of Mur. N". 1 

 with 38 pairs of muscle segments, pli = pericardial cavity, m = myoeard, e = endo- 

 card, per = periblast, ent = entoderm, cli = chorda, oes = oesophagus. Enl. = 240. 



Fig. 2. Two cells of the myoeard of an embryo of the same species of Mur. 

 N'\ 1 with 44 muscle segments. Longitudinal section, sublim.-formol, iron-haema- 

 toxylin and eosin. Enlarg. = 800. 



Fig. 3. The same, cut tangentially. Enlarg = 800. 



Fig. 4. Cross section through the heart of a slightly older embryo. 



Fig. 5. Surface section of the wall of the atrium of a larva of Mur. N". 1, 

 five days old. 



Fig. 6. Section through the wall of the ventricle of a larva of salmo fario of 

 22 mM. Enlarg. =800. 



