Simultaneously, vein formation occurs, and the veins join 

 into two main stems in the posterior and anterior halves 

 of the vascular field; both stems enter the left bend of 

 the cardiac canal. The posterior part of the heart forms 

 a horseshoe- shaped elevation directed to the right. At 

 that time the heart does not yet appear in the region of 

 the future chest cavity, but in the region of the neck. 



The second period of vascular development is charac- 

 terized by a circular blood movement, which is connected 

 with the blastoderm. The blood moves primarily anteriorly 

 and in two great stems is transferred into one or two 

 anterior veins of the blastoderm. From the posterior part 

 of the blastoderm, blood collects initially in one (left) 

 posterior vein of the blastoderm, and only later the right 

 posterior vein appears. All venous blood enters both bends 

 of the heart, but mainly the left. From the two stems of 

 the aorta go the posterior vertebral arteries and the 

 blastoderm arteries. The latter, branching, reach the 

 border sinus. In the second period the vessels of the 

 blastoderm become vessels of the yolk sac. 



The blood-carrying system at this stage has been 

 demonstrated in a distinct and refined schematic drawing, 

 with letter designations as mentioned below in the text. 8 

 (Drawing 29) The vascular system of the embryonic body is 

 composed of the following vessels. The vein of the yolk sac 

 N begins in the mesentery; both arteries of the blastoderms 

 join to make a common stem, the artery of the yolk sac p. 

 The embryonic veins pour into a vessel which passes between 

 the liver and heart and produces the liver branchings (the 

 portal system) . The veins going from the head along the 

 lateral sides of the neck collect blood from the brain and 

 neck region (the anterior vertebral veins g) and pour into 

 a common venous stem o. From the posterior end of the body, 



8. The relationship of Baer's idea by the vascular system 

 in the amniotes to the contemporary data given by 

 P. G. Svetlov in the note to the Russian translation of 

 the second volume of HISTORY OF ANIMAL DEVELOPMENT 

 (see note 85 on pp. 468 - 469) . 



398 



