the membrana vasculosa gives rise to the heart 

 and blood vessels; and the membrana serosa is 

 the source of the spinal cord, sides of the body, 

 and amnion. All of this is made clearer by the 

 drawings and descriptions given here. 36 

 (Figure 24) 



Pander explained the blood system as corresponding to 

 the four stages of its embryonic formation. The first stage, 

 illustrated in Table VIII of the German version, leads to 

 the formation of three blood passages: along the circle at 

 the border of the vascular area (the border vein) , along the 

 branches connecting the border vein with the fetus, and along 

 the vascular stem in the main body of the fetus. The upper 

 or the descending vein (usually double) appears in the form 

 of a stem from the heart-shaped curve of the border vein. 

 The lower or the ascending vein always begins with many 

 branches, passes above the arterial stem and, taking many 

 lateral branches, connects with the upper vein near its 

 entrance in the heart. 



The second stage of blood-circulation development is 

 characterized by gradual disappearance of the upper and 

 lower veins. For this, particularly on the fifth day, veins 

 develop and pass along the route of the arteriesj however, 

 the arteries are under the veins. This structure remains 

 to the ninth day, and by the fifteenth day of incubation the 

 vessels of the blastoderm become insignificant and then 

 disappear. The arch of the aorta is formed from two or 

 three branches which go out from the bulb (76) . In the 

 region of the heart depression, this single stem divides 

 into two equal arterial stems, descending in parallel 

 along both sides from the spinal column to the tail. The 

 arteries going out from them at right angle Pander considered 

 vessels corresponding to the iliac arteries of the develop- 

 ing animal. This, of course, is not accurate; this is the 

 umbilical-mesenteric artery. Later all three veins join in 

 the heart, which they enter by a common stem. 



36. Ibid., p. 515. 



266 



