THE PRIMITIVE HEART. 



-05 





DEVELOPMENT OF THE HEART. 



In the mammals in which the earliest stages in the development of the heart have 

 been observed, this organ arises as two separate tubes that are formed by folding of 

 the visceral mesoblast near the margin of the embryonic area. This folding occurs 

 while the embryo is still spread out upon the surface of the yolk-sac and produces on 

 each side an elevation, a heart-tube, that projects into the primitive body-cavity 

 (Fig. 668). Each heart-fold differentiates into a thicker outer or myocardial layer, 

 which gives rise to a portion of the cardiac muscle, and a very thin inner endocardial 

 layer, from which the serous lining of the heart is derived. The latter consists of a 

 single stratum of mesoblastic cells surrounded by the muscle-layer, but separated by 

 a distinct space, as a shrunken cast lies within its mould. 



With the beginning constriction of the gut-tube from the vitelline sac and the 

 associated approximation of the splanchnopleura of the two sides, the heart-tubes, at 

 first widely apart, gradually approach the mid-line until they meet beneath the ventral 

 surface of the primitive pharynx, in advance of the yolk-sac. Upon coming into 

 contact, the cavities of the two heart-tubes for a brief period remain separated by the 

 partition formed by the opposed portions of the myocardial layers. Very soon, how- 

 ever, solution of this septum occurs and the two sacs become a single heart. The 

 endothelial tubes are last to fuse, retaining their independence after the myocardial 

 walls have blended. Upon fusion of the endothelial layers the conversion of the double 

 tubes into a single heart is 

 complete. Fig. 668. 



The early venous trunks 

 — the body-stems (cardinals 

 and jugulars) within the em- 

 bryo and the vitelline and 

 allantoic (later umbilical) veins 

 from the extra embryonic vas- 

 cular net-works — converge to- 

 wards a common sac, the 

 sinus ve7iosus, which joins the 

 caudal end of the cylindrical 

 primitive heart. The slightly 

 tapering cephalic extremity of 

 the latter becomes the trnncus 

 arteriosus, from which two trunks, the ventral aortcs, are prolonged forward beneath 

 the primitive pharynx, giving off the aortic bows that traverse in pairs the series of 

 visceral arches. The primitive heart consists, therefore, of a cylinder, somewhat 

 contracted at its anterior end, that occupies the ventral mid-line in the later cervical 

 region. The blood poured into the sinus venosus by the veins enters its posterior 

 extremity and escapes anteriorly through the truncus arteriosus. 



Although for a brief period the heart-tube retains its median position and 

 straight cylindrical form, its increasing length soon causes it to become bent upon 

 itself and to assume the S-like contour shown in Fig. 672 A, from an embryo of 2. 15 

 mm. in length, in which the venous end of the tube lies below and to the left and 

 the arterial trunk above and to the right. The intermediate portion of the tube, 

 extending at first downward and then obliquely upward and towards the left, is the 

 primitive ventricle, the early sigmoid heart-tube already suggesting the recognition 

 of an arterial, ventricular, and venous segment. 



During the further development of the heart a rearrangement of these three 

 divisions takes place, since the venous segment, orginally below, gradually acquires 

 a position above and behind, while the primitive ventricle comes to lie in front and 

 below (Fig. 672). With the completion of this rotation, a deep external groove 

 appears between the ventricular and venous chamber, now the. primitive auricle, that 

 indicates the position of a contracted passage, the auricular canal (Fig. 672, C ), as 

 the common auriculo-ventricular opening is termed. Coincidently with the upward 

 migration of the venous segment, a lateral outpouching of the auricular chamber 

 appears on each side of the truncus arteriosus. These expansions, the primary au- 



45. 



Myocardial 

 layer 

 Endocardial layer 



Gut-tube 



Splanch- 

 nopleura 



Heart-tube 



Transveise section of very young rabbit em- 

 bryo, showing two heart-tubes widely separated by 

 unclosed digestive tube. X 150. 



