60 H. VON W. SCHULTE 



iilar sulcus is still oblique. It is only as the venous end of the 

 heart begins to move towards the right that the septum assumes 

 a dorso-ventral direction. Its transverse direction in early 

 stages is rendered possible by the primary displacement of this 

 extremity to the left as will be demonstrated in the considera- 

 tion of the endothelial analges. 



To summarize, the fusion of the myoepicardial mantles is 

 accomplished with the aid of a middle cardiac plate, which sub- 

 sequently becomes reduced to a ridge marking the primitive 

 median line during the formation of the loop. It is continued 

 caudad as the interventricular septum, which thus forms in 

 the line of original fusion of the heart anlages and by its ap- 

 pearance separates again, so far as the ventricles are concerned, 

 the primitive bilateral anlages. The myocardium in the early 

 stages of fusion is bilaterally symmetrical with a well marked 

 bulbo-ventricular sulcus on each side. In the formation of the 

 loop the left sulcus deepens and the right opens up and gradual- 

 ly is obliterated. It is possible that the middle plate now re- 

 duced to a ridge and located at the convexity of the forming 

 loop, is less plastic than the thinner portions of the mantles and 

 failing to lengthen to a sufficient degree exerts a traction which 

 occasions the appearance of the interventricular sulcus. 



THE ENDOTHELIAL TUBES 



The origin of the endocardium differs in nowise from the ori- 

 gin of endothehum elsewhere in the cat. It develops from mes- 

 enchyme which is formed in loco, first by migration of cells from 

 the compact visceral mesoderm, second by delamination of 

 groups of cells from the same source. In this process ridges 

 and projections are formed by the mesoderm from which the 

 mesenchyme loosens itself. A means of migration is afforded 

 the amoeboid cells by the presence of interdermal cytodesmata, 

 delicate protoplasmic bridges stretching between the mesoderm 

 and entoderm. The early mesenchyme consists of single cells 

 and scattered groups which are arranged in plates or even 

 cords. Within these groups vacuoles appear and enlarging flat- 



