PHYSIOLOGY OF THE EMBRYO 317 



minalis, while veins proceed from the sinus to the heart. 

 The system of vessels thus formed is called the vascular 

 area. Shortly after the heart is formed it begins to beat 

 rhythmically, thereby setting in circulation the fluid formed 

 in the vascular system. It is worthy of note that the cardiac 

 muscle contracts rhythmically at a time when it contains no 

 ganglionic cells. 



By means of the vitelline circulation the embryo is sup- 

 plied with nourishment which has been taken up by the 

 blood from the yolk-sac. 



The red blood corpuscles originate from the so-called 

 blood islands, i.e. groups of cells in the cords from which 

 the blood vessels are developed. The cells of the blood 

 islands form blood pigment, separate, and then appear as 

 nucleated blood corpuscles suspended in the fluid. 



(2) Placental circulation. From the abdominal aorta 

 formed by the union of the primitive aortic arches proceed 

 the two umbilical arteries through the umbilical cord (the 

 Avail of the allantois) to the place where the allantois joins 

 the chorion and where the placenta originates. Here the 

 arteries split up into capillaries. From these capillaries 

 the blood is collected by the umbilical vein, which passes 

 through the umbilical cord to the navel; thence, as the 

 ductus venosus Arantii, under the liver to the inferior vena 

 cava. 



At this time the right and the left heart are not yet com- 

 pletely separated. In the septum between the auricles exists 

 an aperture, the rafaula Eustachii. The pulmonary arteries 

 and the aorta are still united by one of the primitive aortic 

 arches,"" the so-called ductus Botalli. Part of the blood from 

 the right auricle passes, therefore, through the valvula Kus- 

 tachii directly into the left auricle and thence into the left 

 ventricle and aorta, while part of it passes from the right 



* Corresponding to the live pairs of gill-arches, tive pairs of aortic arches are 

 formed which undergo the following changes : The first two pairs disappear; tin- 

 third pair forms the external carotids; the fourth arch on the left side tonn> the 

 aorta, on the right side, the right subclavian; the fifth on the 4eft side, the ductus 

 Uotalli and the left pulmonary artery ; on the right side the right pulmonary 

 artery. 



