beginning appear close together. The current of blood from 

 the right ventricle moves into the fifth left arch more than 

 in the fourth. From the fourth left ventricle, it passes 

 into the fourth left arch more than in the fourth right. 

 Therefore, in mammals the movement of blood in the left 

 side is more intensive, and the arch of aorta forms from 

 the left root, while in birds it is from the right. 



In a footnote Baer objected to Allen Thomson's scheme 

 (II cc, pp. 212-213). The latter had taken Baer's drawing 

 of the blood vessels in a chick embryo, transferred the 

 right parts to left and vice versa, and proposed that it is 

 thereby possible to represent the vessels of the mammalian 

 embryo. To prevent such erroneous interpretations, Baer 

 made a new drawing (Figure 29, 14) of the mammalian trans- 

 formation of the branchial vascular system into constant 

 arteries. This drawing makes clear that from the anterior 

 bifurcated end of a primarily single arterial trunk, five 

 pairs of vascular arches proceed to the two roots of aorta 

 b. Early disappearing arches are represented by a dotted 

 line, and those remaining are represented by a thin contour, 

 while the final vessels are black. In the venous system 

 of the mammalian embryo, the same vessels which appear in 

 birds are present. Vessels of the yolk sac and the boundary 

 vein are also present. The coincidence between birds and 

 mammals is especially great with the venous vessels of the 

 embryo itself. In those and others, there are anterior and 

 posterior vertebral veins. The posterior vein trunk in 

 mammals, as in birds, unites with the posterior veins of 

 the body; after that the diameter of the vertebral veins 

 decreases. The jugular veins only at first are connected 

 with the anterior vertebral veins and then are separated 

 from it. 



The formation of the neural tube and differentiation of 

 its anterior part into five cerebral sacs takes place in 

 mammals as in birds. However, in mammals the middle cerebrum 

 is not so high and sac-formed, while in birds it is more 

 elongated and curved. Unlike that in birds, the predominance 

 of the anterior cerebrum over the other parts appears very 

 clearly in mammals. But the head cerebrum, is more bent in 

 mammals than in any other class of vertebrates. The trans- 

 formation of the spinal cord to the medulla oblongata takes 



413 



