DEVELOPMENT OF THE HEART. 



145 



Fig. 174. DIAGRAM SHOWING THE 



DIVISION OF THE LOWER PART OP 

 THE BULBUS AORTJS, AND THE FOR- 

 MATION OF THE SEMILUNAR VALVES. 



(After Gegeiibaur and His. ) 



A, undivided truncus arteriosus 

 with four endocardia! cushions; B, 

 advance of the two lateral cushions 

 resulting in the division of the lumen ; 

 C, projection of three endocardia! 

 cushions in each part ; D, the separa- 

 tion into aorta and pulmonary trunks 

 completed. 



Fig. 175. SECTIONS THROUGH THE 



HEART OF HUMAN EMBRYOES, SHOW- 

 ING TWO STAGES IN THE FORMATION 

 OF THE CARDIAC SEPTA AND OF THE 

 AURICULO-VENTRICULAR VALVES. 



(His.) 



A, from an embryo of 5 or 6 weeks. 

 R. V, right auricle ; L. V, left auri- 

 cle ; S.r.d, right horn of sinus ; S.r.s, 

 left horn of sinus ; V.E, Eustachian 

 valve ; s.int, septum superior and 

 endocardial cushion (septum interme- 

 dium, His) ; s.inf, septum inferius 

 ventriculorum. This septum, as well 

 as the bulk of the ventricle, is a mus- 

 cular sponge at this stage. Oe, oeso- 

 phagus ; Br, bronchus. 



B, from a somewhat more advanced 

 embryo. Ad, As, right and left 

 auricle ; Ost, auriculo-ventricular 

 apertures; S.s. septum superior of 

 auricles ; S.it, endocardial cushion 

 (septum intermedium) ; S.if, septum 

 inferius ventriculorum, now denser 

 and more muscular ; Mpp, peri- 

 cardial attachment. 



Distinct muscular tissue is 

 seen in the cardiac wall, even 

 as early as the stage of an 

 S-shaped tube, although the 

 heart begins to pulsate regu- 

 larly long before this. The 

 muscular layer is separated 

 from the epithelial lining of 

 the cavities (endothelial tube 

 of His) by a layer of clear 

 gelatinous tissue,bridged across 

 by fine fibres (embryonic con- 

 nective tissue). This layer is 

 most abundant in the ventri- 

 cular part and aortic bulb, and 

 here the endothelial tube is 

 consequently much smaller 

 than the muscular tube. Sub- 

 sequently, in the ventricle, the 

 gelatinous tissue is invaded by 

 muscular bands which grow 



VOL. I. 



