FUSION OF CARDIAC ANLAGES IN THE CAT 53 



Embryos of 11 pairs of somites show some variation in devel- 

 opment. Some lia\'o hardly progressed beyond the one of 9 

 somites, Init in the most advanced of this stage the mantles are 

 approximated in the greater part of their length and the middle 

 plate is considerablv reduced. Fusion has not yet occurred 

 (fig. 3). 



The mantles present two angulations in thier ectal contour. 

 The first bend is the bulbo-ventricular sulcus already described, 

 which becomes accentuated on the left side in the later stages 

 as the flexion of the tube develops. In this embryo it is near- 

 ly rectangular and is approximately symmetrical on the two 

 sides. I take the deep incisure in Martin's cat embryo of 4 mm. 

 to be its equivalent. It is also to be recognized in Duval's fig- 

 ure'' of a chick of 8 somites, though obscured by the outline of 

 the amnion. The second bend is a more gradual change of 

 direction at the junction of the heart tube with the omphalo- 

 mesenteric vein and may accordingly be designated the cardio- 

 venous angle. It is not to be distinguished in Martin's figure, 

 but in the heart of the chick as shown by Duval it is a deep 

 cleft. These angulations are important in that they determine 

 the earliest points of fusion in later stages between the endothe- 

 lial tubes. 



The middle plate is much reduced. Its cephalic portion is 

 concealed in the deep cleft between the bulbs; caudad it forms a 

 convex triangle exposed in its whole extent. Throughout the 

 mantles are markedly dorso-ventrally flattened. 



The fusion between the mantles and the formation of a dorsal 

 mesocardium is effected in embryos of 12 and 13 pairs of somites. 

 The dorsal mesocardium is very short, set off by a sulcus from 

 the mesoderm covering the foregut and ventrally by a deeper 

 sulcus from the mantles. Cephalad its leaves separate to give 

 passage to the forming aortic roots and caudad it expands and 

 is continuous with the lateral mesocardia. 



^ Both these cuts are given by Mollier. Op. cit. figures 713 and 715. Allen 

 Thompson illustrates this form of the heart in the chick in his article 'On the 

 development of the vascular system in the foetus of vertebrated animals.' Edin- 

 burgh new philosophical journal, 1830, PI. 2, fig. 12. 



