DEVELOPMENT OF THE FECUNDATED EGG. 515 



completed by a development of a trunk which collects the 

 blood that comes from the undeveloped lower limbs. The 

 ductus venosus (canalis Arantii) and the sinus venosus are 

 formed of that portion of the umbilical and mesenteric veins, 

 which lies between the confluence of the afferent hepatic and 

 efferent hepatic veins (Fig. 147, B and C, 4). 



We will not insist upon the ulterior results of this 

 arrangement, which is one of the most complicated points 

 in the anatomy of the liver. It is only necessary to under- 

 ftand that the umbilical vein, when it arrives at the liver, 

 pours a part of its contents into the portal vein (into the 

 left portion of the portal vein), and conveys another portion 

 through the ductus venosus directly into the lower vena 

 cava, and thence to the heart. 



The veins which collect the blood from the body of the 

 embryo (anterior, or superior, and posterior, or inferior, cardi- 

 nal, and lower vena cava, see Fig. 148) empty their contents 

 simultaneously and on each side, into a common duct (canalis 

 Cuvieri). But this arrangement does not long continue ; for 

 soon the posterior, or inferior, cardinal veins become partially 

 atrophied, and the only trace of their existence is found in 

 the azygos veins (large and small azygos, see Fig. 149, B). 

 Between the anterior cardinal veins a transverse duct (left 

 brachio-cephalic, 7, A and B, Fig. 149) is formed, simulta- 

 neously with the atrophy of the left Cuverian duct. On the 

 other hand, the right duct of Cuvier persists and becomes 

 the superior vena cava (Fig. 149, A, 6). We thus understand 

 the arrangement of the right azygos vein (large azygos) 

 which in the adult conducts the blood into the superior vena 

 cava, since it represents the central end of the right posterior 

 cardinal vein ; as well as the arrangement of the right brachio- 

 cephalic trunk, which represents the central end of the right 

 superior cardinal vein. At this period of embryonic existence 

 the inferior and superior vena3 cava) empty their contents into 

 the heart by a common trunk, while at a later period of its 

 existence this common trunk gradually becomes a portion of 

 the wall of the auricular cavity ; so that, after a while, the 

 two vena? cavaB separately connect with the auricle (as in the 

 adult) at a little distance from each other. 



b. Heart. The central organ of circulation, at first repre- 

 sented in the form of a simple and cylindrical tube, after- 

 wards resembling the letter S (Fig. 146), becomes divided by 

 means of increasing constrictions into three cavities, viz., the 

 auricular, ventricular, and arterial (aortic bulb or sinus, 



