the blood flows in the vessels, passing at the upper border 

 of the mesenteric plates. In these vessels (posterior 

 vertebral veins hi) veins pour in from the tail h and 

 posterior extremities, cloaca, pelvic region, the posterior 

 end of the kidneys and small branches of each intervertebral 

 space. The general stems, in which the anterior and 

 posterior vertebral veins join, are named the umbilical 

 venous stems K; both these stems go to the heart ab. Only 

 later is the posterior hollow vein m separated. From the 

 union of the vessels going along the lower border of each 

 abdominal plate, in the third period the umbilical vein 

 forms . 



The changes in the heart in the second period are as 

 follows. From the posterior part of the cardiac canal 

 the common venous stem forms, from the anterior part, the 

 arterial stem, and from the middle, the whole heart chamber. 

 Behind the passage from the venous stem, two sacs protrude 

 into the heart chamber; these are the nondivided auricles. 

 In the cavity of the middle part of the cardiac canal, 

 from the convex side a lower fold grows inside and separates 

 the single blood current into two. The arterial division 

 of the heart widens into the trunk of the aorta. The first 

 branchiate vascular arch disappears on the fourth day, and 

 in replacement the fifth arch develops. Simultaneously, in 

 the place of the union of the first arterial arch with the 

 spinal aorta, the rudiment of the vertebral artery d is 

 formed; and from the place of union of the first arch with 

 the stem going from the root of the aorta, the cephalic 

 artery c forms. The spinal aorta at the posterior end of 

 the kidney divides into two vertebral arteries. "For blood 

 circulation of the second period, it is characteristic that 

 the blood in the whole route does not pass through the differ- 

 entiated organs of respiration," Baer asserted (II 7gg, 

 p. 188 (132)) . 



The third period is characterized by blood circulation 

 through the external respiratory organ, whose role is 

 responding to the quickly growing urinary sac. The branches 

 of the aorta going into it, the umbilical arteries, enlarge. 

 Consequently, the umbilical veins also enlarge, especially 

 the left vein and the posterior hollow vein. The posterior 

 vertebral veins are reduced into an unpaired vein. The 



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