754 THE CIRCULATORY SYSTEM 



begins to travel forward above the ventricle (figs. 336C; 339H'). At the end 

 of the third day, the heart attains the typical sigmoid or S-shaped condition 

 which arises as the first major step in heart development in all vertebrate 

 embryos. During the fourth day of incubation, the atrial area expands into 

 two main lobes, the beginnings of the right and left atria; the ventricular 

 area expands greatly and thickens; and the bulbus cordis lies in the median 

 line between the developing atria (fig. 3391). The position of the various 

 parts of the heart on the whole assumes more nearly the adult condition. 



Internally, toward the end of the fourth day, an interatrial septum begins 

 to develop from the dorso-anterior area between the two atrial lobes, slightly 

 to the left of the opening of the sinus venosus. The septum continues to form 

 posteriad toward the narrowed atrio-ventricular opening between the atria 

 and the forming ventricles. Simultaneously in the atrioventricular opening, two 

 endocardial thickenings, the endocardial cushions, arise, one dorsal and one 

 ventral. At the apex of the ventricle, an interventricular septum appears and 

 grows forward toward the atrioventricular opening (fig. 340G). 



During the fijth and sixth days, the two endocardial cushions grow together 

 and separate the atrioventricular canal into two passageways by the formation 

 of a cushion septum. The atrial septum grows toward the endocardial cushion 

 area and unites with the cushion septum. However, the atrial septum never 

 is completed during embryonic life, as small openings or fenestrae, appear in 

 the septum permitting blood to pass through the septum. During the last week 

 of incubation, the fenestral openings in the atrial septum become much smaller 

 and completely close shortly after hatching. The ventricular septum, mean- 

 while, grows forward to unite with the cushion septum. Up to the fifth day, 

 but one passageway leaves the heart via the developing bulbus cordis and 

 ventral aorta. However, during the fifth day, beginning at the area just anterior 



Fig. 340. Early stages in morphogenesis of various vertebrate hearts (Continued). 

 (A-E) Internal changes in the developing heart of the pig. (A-D, redrawn from Patten, 

 1948. Embryology of the Pig, 3d edit., Blakiston, rn. idelphia.) (A) Diagram of 3.7 

 mm. pig embryo heart, ventral wall removed. (B) Similar diagram of 6 mm. pig heart. 

 (C) Similar diagram of 9.4 mm. pig heart. (D) S'milar diagram of dissected pig fetal 

 heart shortly before birth. (E) Schematic drawing of dis5 ,cted 18 mm. pig heart 

 viewed from right side with walls of right atrium and right v;ntricle removed. Observe 

 that the bulbus cordis has divided into two vascular trunks. (F) Dorsal aspect of the 

 heart of an 11 wk. (60 mm.) human embryo. (Redrawn and modified from Patten, 

 1946. Human Embryology, Blakiston, Philadelphia.) The contraction wave of the heart 

 beat is indicated by heavy arrows. Starting at the sinus node situated in the dorsal wall 

 of the right atrium, the contraction wave spreads over the atrial walls and also to the 

 atrioventricular node located in the atrial septum from whence it travels distally through 

 the ventricular tissue. (G) The developing chick heart, of about 6-7 days. Right walls 

 removed to show developing cardiac septa. The ventricular septum is still incomplete, 

 and the atrial septum is fenestrated. (This figure has been modified considerably from 

 Kerr. 1919. Text-Book of Vertebrate Embryology, vol. II, Macmillan, Ltd., London, 

 after Greil.) (H) Adult heart of the South American lung fish, Lepidosiren paradoxus, 

 right side removed. (Redrawn from Robertson, 1913. Quart. J. Micros. Sci., 59.) 



