DEVELOPMENT OF THE HEART. 381 



(atrium). The latter, like the simple auricle of the heart 

 of the Fish, forms a pair of lateral protuberances, the heart 

 ears, or auricular appendages (auriculae, Fig. 307,6); and 

 hence the indentation between the auricle and ventricle is 

 called the auricular canal (canalis auricularis, Fig. 308, ca). 

 The heart of the human embryo is now a complete Fish 

 heart. 



Corresponding exactly with the Phylogeny of the human 

 heart (Table XLI), its Ontogeny exhibits a gradual tran- 

 sition from the Fish heart through the Amphibian heart to 

 the Mammalian heart. The most important step in this 

 advance is the formation of a longitudinal partition, im- 

 perfect at first, afterwards complete, by which all the three 

 sections of the heart are separated into a right (venous) and 

 a left (arterial) half. (Cf. Figs. 309-314.) The auricle 

 (atrium) is thus divided into a right and a left auricle, each 

 of which acquires its respective auricular process ; the body- 

 veins discharge into the right auricle (ascending and de- 

 scending vena caves, Fig. 311, c, Fig. 313, c) ; the left auricle 

 receives the lung- veins. Similarly, a superficial "inter- 

 ventricular furrow" (sulcus interventricularis, Fig. 312, s) 

 appears at an early period on the main chamber of the 

 heart, the external expression of the internal partition, by 

 the formation of which the ventricle is divided into two 

 chambers, a right (venous) and a left (arterial) ventricle. 

 Finally, a longitudinal partition forms, in a similar way, 

 in the third section of the primitive heart, which so much 

 resembles that of a Fish, in the arterial stalk, which is also 

 externally indicated by a longitudinal furrow (Fig. 312, af). 

 This separates the cavity of the artery-stalk into two 

 lateral halves ; the main lung artery, which opens into the 



