750 THE CIRCULATORY SYSTEM 



c. Importance of the Septum Transversum to the Early Heart 



There is another structure which is important to the primitive embryonic 

 heart and to its later development. This structure is the primary septum 

 transversum or the mesodermal partition which forms across the coelomic 

 cavity, below (ventral) to the lateral mesocardia. It forms not only a par- 

 tition or bulwark, separating the developing liver substance from the primitive 

 heart, but it is also a suspensory ligament for the caudal end of the sinus 

 venosus and the converging veins of the heart. (See Chap. 20.) 



d. Activities of Early-Heart Development Common to All Vertebrates 



The early stages of heart development, following the formation of the basic 

 rudiments mentioned above, are essentially the same for all vertebrates. 

 These changes, which result in the formation of a sigmoid or S-shaped struc- 

 ture, are as follows (see figs. 336, 339): 



(1) The dorsal mesocardium soon disappears for most of its extent, and 

 the primitive heart tube begins to elongate and to change its shape 

 rapidly. 



(2) The ventricular portion bends ventraliy and to the right and, at the 

 same time, grows posteriad, becoming thick-walled. 



(3) The atrial area expands laterally, grows forward dorso-anteriad over 

 the ventricular area; and at the same time forms two lateral lobes. 



(4) The sinus venosus remains thin walled and rigidly attached to the 

 septum transversum. The latter, in all vertebrates above the fishes, 

 bends forward along its upper margins during the early period of 

 development. 



(5) The bulbus cordis extends slowly and becomes a thickened anterior 

 continuation of the heart from which arise the ventral aortic roots. 



e. Development of the Heart in Various Vertebrates 



From the generalized, S-shaped, basic condition, the hearts of the various 

 vertebrate groups begin to diverge in their development as follows: 



1) Shark, Squalus acanthias. Starting as a straight tube when the embryo 

 is 5.2 mm. long (fig. 339A), the ventricular portion begins to bend toward 



Fig. 339. Early stages in morphogenesis of various vertebrate hearts. (A-C) Stages 

 in heart development in Squalus acanthias. (Redrawn from Scammon, 1911, Chap. 12, 

 in Normentafeln Entwichlungsgeschichte der Wirbeltiere by F. Keibel, G. Fischer, Jena.) 

 (D-F') Heart development in the frog, Ratui pipiens. (D-F) Left lateral views; (F') 

 ventral view. (G-K) Heart development in the chick, ventral views. (H-K, redrawn 

 from Kerr, 1919, Text-Book of Embryology, vol. II, Macmillan and Co., Ltd., London, 

 after Greil.) (L-O) Heart development in the human embryo, ventral views. (Redrawn 

 from Kramer, 1942, Am. J. Anat. 71. L, after Davis, modified; M, after Tandler, 

 modified; N, after Waterston, modified.) Observe that ventricular end of the original 

 bulbus cordis, i.e. the conus portion, contributes to the right ventricle in diagrams N and O. 



