DEVELOPMENT OF THE HEART. 



137 



mammal shows distinct indications of division into its several parts ; indeed these 

 parts are apparent even while the two tubes are still distinct, as the accornpan vino- 

 sketches of the rabbit's heart clearly show (fig. 159). The heart does not, however, 



Fig. 162. TRANSVERSE SECTION THROUGH 



THE REGION OP THE HEART IN A RABBIT 

 EMBRYO OF NINE DAYS, SHOWING THE 

 COMMENCING FUSION OF THE TWO TUBES. 



'. (Kolliker.) 



jj, jugular veins ; ao, descending aorta? ; 

 ph, pharynx ; hp, epiblast of body-wall ; 

 ik, endothelial lining of the still divided 

 heart ; ah, outer wall of the heart ; p, peri- 

 cardial coelom ; df, df, visceral mesoblast 

 (somatopleure) ; e', prolongation of the 

 hypoblast of the foregut and the anterior 

 wall of the pericardial cavity into the 

 partition between the two halves of the 

 heart ; bl, bilamiriar portion of blastoderm 

 forming pro-amnion ; ect, ent, its two 

 layers (epiblast and hypoblast). 



cct 



an in 



long retain its symmetrical position. It soon becomes bent upon itself, so as to 

 assume the shape of an S, the anterior part of the tube bending over to the right and 

 the posterior to the left (fig. 164, B). At the same time the posterior, or sino-auricular 



Fig. 163. SECTION THROUGH THE REGION OF THE HEART IN A RABBIT EMBRYO OF 10 DAYS, AFTER 



THE TWO TUBES HAVE UNITED INTO A SINGLE MEDIAN ORGAN. (Kolliker. ) 



ao, descending aortse ; la, bulbus aortse ; ah, its external wall ; mp, posterior mesocardium, uniting 

 the heart to the ventral wall of the pharynx, ph, and here separating the pleuropericardial ccelom, 

 p. into two halves, which are, however, united on the ventral side of the heart ; ent, hypoblast of yolk 

 sac ; df, its mesoblast ; df, mesoblast of pharynx ; ect, epiblast. 



Fig. 164. OUTLINES OF THE ANTERIOR HALF OF THE EMBRYO CHICK VIEWED FROM BELOW, SHOWING THE 



HEART IN ITS EARLIER STAGES OF FORMATION. (After Remak. ) 2 f 



A, embryo of about 28 to 30 hours ; B, of about 36 to 40 hours ; a, anterior cerebral vesicle ; 

 b, proto-vertebral segments ; c, cephalic fold ; 1, 1, vitelline or omphalo-mesenteric veins entering the 

 heart posteriorly ; 2. their union in the posterior part of the heart ; 3, the middle part of the tube 

 corresponding to the ventricle ; 4 (in B) the arterial bulb. 



end of the heart, gradually comes to lie behind or dorsal to the ventricular part, 

 which arches transversely from left to right, where it turns sharply upward (towards 

 the head), and terminates in the bulb. The tube is divided by slight constrictions 

 into successive portions, viz. : (1) the part formed by the junction of the principal 

 veins, sinus venosus ; (2) the auricular part ; (3) the ventricular part ; and (4) the 

 aortic bulb. 



