( 221 ) 



diieed drawing serving only (o show the l()p()gi';i|>liit' relations. It is 

 only the niyocard llial interests ns here. The cells of those parts of' 

 the Avails of the pericardial cavity that form the niyocard, are di- 

 stinctly separated at this stage of development, are of a cnbical or 

 cylindrical shape and very regular, as is shoAvn in the figure. They 

 possess a rather large round nucleus and have a granular looking 

 protoplasm which shows no definite structure organisation. At both 

 ends of the heart tube they gradually diminish in height until the 

 flat shape of the cells of the other parts of the pericardial plates 

 is reached. 



The cell boundaries between the heart muscle cells are everywhere 

 sharply delined ; in pre})arations stained with iron-haematoxylin, at 

 both sides of the heart tube (that turned towards the endocard and 

 that turned towards the pericardial cavity) a delicate black line, 

 following the cellular limits, is to be seen — the "Schlussleiste". 



The first signs of differentiation, which showed themselves in the 

 heart muscle cells, tend already to give rise to a fibrillar structure. 

 A granular stage, as described by Godlewski, during which the proto- 

 plasm of the cells is full of deeply staining granules, which arrange 

 themselves in rows and fuse to give rise to the myofibrillae, I have 

 not been able to find. On the stage of the granular looking incsh- 

 work with small meshes, the usual appearance of protoplasm, there 

 folloAved in my preparations immediately a stage, in which at the 

 basal end of the cell (viz. that turned towards the endocard), extremely 

 delicate fibrillae are to be made out, which in most cases run at 

 right angles to the longitudinal axis of the heart tube. 



These fibrillae, as far as could be made out, are homogeneous 

 from the beginning, and do not give the impression of being composed 

 of or derived from granules arranged in rows. However, this need 

 not lead ns to doubt the formation of the fibrillae in this way 

 even here; the beautiful figures and clear descriptions of Godlewski 

 are too convincing on this point. Perhaps this stage lasts only a short 

 time and is not represented in my preparations which are stained 

 with iron-haematoxylin. Be this as it may, we only find the extre- 

 mely delicate fibrillae ^), which thicken and become more distinct during 

 the foUoAving stage. To this stage belong the sections drawn in tig. 

 2 and fig. 3 (longitudinal sections) and fig. 4 (cross section through 

 the heart tube). 



In order to understand these drawings rigiilly, the following may 

 be of use. The heart is during this stage still lying as a straight 



1) A regular network consisting oC large |)rotoplasmic discs as described by Mc 

 Gallum, 1 have never been able lo find. 



15* 



