DEVELOPMENT. 799 



feriorly. The heart of fishes retains these cavities, no further division 

 by internal septa into right and left chambers taking place. In 

 amphibia, also, the heart throughout life consists of the three muscular 

 divisions which are so early formed in the embryo and the sinus venosus; 

 but the auricle is divided internally by a septum into a pulmonary and 

 systemic auricle. In reptiles, not merely the auricle is thus divided into 

 two cavities, but a similar septum but incomplete is more or less developed 



Fig. 494. Heart of the chick at the 45th, 65th, and 85th hours of incubation. 1, the venous 

 trunks; 2, the auricle; 3, the ventricle; 4, the bulbus arteriosus. (Allen Thomson.) 



in the ventricle. In birds and mammals, both auricle and ventricle 

 undergo complete division by septa ; while in these animals as well as in 

 reptiles, the bulbus aortse is not permanent, but becomes lost in the ven- 

 tricles. The septum dividing the ventricle commences at the apex and 

 extends upward. The subdivision of the auricles is very early fore- 

 shadowed by the outgrowth of the two auricular appendages, which 

 occurs before any septum is formed externally. The septum of the 

 auricles is developed from a semilunar fold, which extends from above 

 downward. In man, the septum between the ventricles, according to 

 Meckel, begins to be formed about the fourth week, and at the end of 

 eight weeks is complete. The septum of the auricles, in man and all 

 animals which possess it, remains imperfect throughout foetal life. When 

 the partition of the auricles is first commencing, the two venae cavae have 

 different relations to the two cavities. The superior cava enters, as in 

 the adult, into the right auricle ; but the inferior cava is so placed that 

 it appears to enter the left auricle, and the posterior part of the septum 

 of the auricles is formed by the Eustachian valve, which extends from 

 the point of entrance of the inferior cava. Subsequently, however, the 

 septum, growing from the anterior wall close to the upper end of the ven- 

 tricular septum, becomes directed more and more to the left of the vena 

 cava inferior. During the entire period of foetal life, there remains 

 an opening in the septum, which the valve of the foramen ovale, devel- 

 oped in the third month, imperfectly closes. 



The bulbus arteriosus, which is originally a single tube, becomes 

 gradually divided into two by the growth of an internal septum, which 

 springs from the posterior wall, and extends forward toward the front 

 wall and downward toward the ventricles. This partition takes a some- 

 what spinal direction, so that the two tubes (aorta and pulmonary artery) 



