50 H. VON W. SCHULTE 



together result in such a remodelling as is hardly to be attrib- 

 uted to unequal growth. In this shortening is to be found an 

 explanation of the kinking of the heart tubes prior to their fu- 

 sion. In the second place the acceleration of infolding of the 

 splanchnopleure cephalad tends to bring the cardiac anlages into 

 an oblique position with their arterial ends near together, their 

 venous extremities divergent. The supposition of accelerated 

 growth in the cephalic portion of the retrocardiac plate is not sup- 

 ported by a conspicuous increase of mitotic figures, but the per- 

 ceptible diminution of its thickness and its rectilinear position 

 at the end of the process suggest the action of a moulding force. 

 Finally MoUier's comment must be borne in mind — the closure 

 of the gut is not the cause of fusion between the cardiac anlages. 

 It determines their approximation, but when this is complete the 

 mantles are separated by an interval, which is bridged by a plate 

 of mesoderm derived from the precardiac plates. By compres- 

 sion and ultimate absorption of this into the mantles fusion is 

 accomphshed, and the process as a whole is evidently one of re- 

 modelling which cannot be explained simply as a cessation of 

 growth in this district. Similar forces may therefore come into 

 play in effecting the early and close approximation of the arterial 

 ends of the mantles notwithstanding the narrowness of the 

 cephalic portion of the retrocardiac plates. 



In an embryo of 9 pairs of somites the cardiac anlages have a 

 position intermediate between those shown in figures I and 3. 

 The mantles are obliquely placed, their arterial ends close to- 

 gether though separated by a deep cleft, their venous ends wide- 

 ly divergent. Each is indented in its lateral contour by an an- 

 gular incisure which marks the junction of their approximated 

 and divergent portions. The differentiation of the heart tube 

 into segments j^rior to fusion was observed by His and the por- 

 tions have been variously designated bulbar and atrial or ven- 

 tricular and atrial. Their subsequent history in the cat show 

 the first segment to be the bulb continuing forward into the 

 truncus, the second the ventricle later expanding at its caudal 

 end into the atrium, but this only after the loop is initiated and 

 fusion well under way. The angulation is therefore the bulbo- 



