134 



DEVELOPMENT OF THE HEART. 



FORMATION OF THE VASCULAR SYSTEM. 



DEVELOPMENT OF THE HEART. 



In mammals, the heart appears in the form of two tubes lying in the cephalic 

 region, one on either side of the embryo. These are seen at a very early period, 

 prior, in fact, to the separation of any part of the alimentary canal from the yolk 

 sac, and to the closure of the neural groove. This bilateral condition was first 

 observed by Hensen in the rabbit ; it has been seen by His in the human 

 embryo. 



The situation and mode of formation of the bi- tubular heart are well illustrated 

 by the accompanying figures from Kolliker. They exhibit the condition in the 



Fig. 157. RABBIT EMBRYO OP THE OTH DAY, FROM THE 

 SURFACE. -, 1 . (Kolliker.) 



The medullary groove is enlarged anteriorly and the primary 

 optic vesicles are growing out from the first cerebral enlarge- 

 ment. On either side of the head, the bilateral tubular heart 

 is seen. Eight pairs of protovertebrse are formed. 



rabbit embryo of about eight or nine days the 

 time when the heart first makes its appearance. 

 Fig. 157 shows such an embryo in surface view. 

 The neural groove, as also the sections show, is 

 widely open, although the rudiments of the cere- 

 bral enlargements are apparent in it, and also the 

 enlargements for the primary optic vesicles. There 

 are eight pairs of proto-vertebrse, the paraxial 

 mesoblast in front of these and on either side of 

 the cerebral enlargements being undivided. Out- 

 side this undivided cephalic mesoblast is a short 

 tube dipping in front into it, and passing behind 

 into a venous trunk, the vitelline or omphalo- 

 meseraic vein of the same side. The tube lies 

 within and is immediately surrounded by a clear 

 space, which is continued forwards beyond it on 

 either side of the fore-brain ; this space is pro- 

 longed from the mesoblastic cleft or pleuro-peri- 

 toneal cavity (ccelom). 



The two short tubes form the double rudiment of the heart. The situation 

 which they occupy becomes, when the lateral walls fold over to form the foregut, 

 the ventral wall of the pharynx, and the two tubes are thus brought together in the 

 middle line underneath the head part of the alimentary canal. Here they soon 

 become fused together to form a single median tube, the hinder end of which is still 

 continuous with the two vitelline veins, while the anterior end bifurcates near the 

 anterior end of the foregut into two branches which arch dorsalwards on either side 

 of that tube, and then pass backwards on each side of the notochord as the two 

 primitive aortse. 



These changes in the position of the primitive heart are partly shown in surface 

 view in figs. 158, 159, but they can only properly be appreciated by the study of 

 transverse sections. Fig. 1GO is a transverse section through the anterior head 

 region of the embryo shown in fig. 157. This is anterior to the heart region, but 

 shows the commencing folding over of the splanchnopleure to form the foregut. 



