752 THE CIRCULATORY SYSTEM 



the right and ventrad in the embryo of 7.5 mm. At 15 mm., the heart appears 

 as indicated in figure 339B, while, at 20.6 mm., it assumes the general ap- 

 pearance of the adult form (fig. 339C). It is to be noted that the ventricular 

 portion of the heart does not bend as dramatically toward the right as in 

 the chick or mammalian heart. In the embryo of 37 mm., the heart already 

 has attained the characteristics of the adult form. The following develop- 

 mental features are present. The bulbus cordis has transformed into the 

 anterior contractile chamber, the conus arteriosus; the ventricular area has 

 developed a pronounced musculature; the atrium is thin walled and bilobed, 

 while the sinus venosus is cone shaped with its base applied against the 

 septum transversum. Right and left valves guard the sinu-atrial entrance. 

 A series of semilunar or pocket valves are arranged around the atrioventricular 

 orifice, while, more anteriorly, cup-shaped valves are forming in transverse 

 rows along the inner walls of the conus arteriosus. 



2) Frog, Rana pipiens. At 4Vi mm. in length, the heart is present as a 

 simple straight tube (fig. 339D). At 5 mm., it begins to bend, the ventricular 

 area moving ventrad and toward the right, and the atrial area and sinus 

 venosus moving anteriad over the ventricular area (fig. 339E). At 7 mm., 

 the heart has assumed the typical S-shaped condition of the adult form, and 

 constrictions appear between the atrium and ventricle (fig. 339F). At this 

 time, also, a median septum begins to divide the atrial chamber. The atrial 

 septum begins as a fold from the antero-dorsal wall of the atrium and grows 

 ventrad and posteriad to divide the atrium into a larger right atrium and a 

 smaller left artium. Moreover, as the atrial septum is developed, it forms to 

 the left of the opening of the sinus venosus into the atrium. Therefore, in 

 the 8- to 10-mm. tadpole, the opening of sinus venosus into the atrium is 

 entirely restricted to the right atrium, and the flow of venous, systemic blood 

 is directed toward the right side of the heart. At about this time, also, the 

 formation of the vena cava inferior proceeds rapidly. (See fig. 335.) At 8 

 to 10 mm., the lung buds (Chap. 14) expand rapidly, and the pulmonary 

 veins begin to bring back blood from the lungs. The pulmonary veins empty 

 into the left atrium (fig. 257B). 



During the late tadpole stages and metamorphosis, internal changes occur 

 which transform the heart into a complicated mechanism, designed to separate 

 and project the oxygenated blood anteriad toward the head and into the 

 systemic vessels; the non-oxygenated blood from the sinus venosus passes 

 into the pulmocutaneous arteries. These different blood currents within the 

 heart are made possible largely by the modification of the internal walls of 

 the primitive bulbus cordis into the highly complicated mechanism of the 

 contractile conus arteriosus. Aside from a series of small pocket valves, the 

 dorsal wall of the conus forms an elongated spiral valve which functions 

 to separate its channel into two parts. The non-oxygenated blood is projected 

 dorsally to the spiral valve and into the pulmocutaneous vessels by the spiral 



