because "of their correspondence with the branchial arches 

 of fish" (I 51, p. 94 (53)) (94). The arches of each side 

 join at the dorsal side of the gullet in one vessel, which 

 Baer called "the root of the aorta." 



At this stage one can already give names to the indi- 

 vidual parts of the vascular system. The veins going from 

 the vascular areas in the abdominal cavity of the embryo 

 are the umbilical-mesenteric veins; they represent the 

 general system of the portal vein. In the embryo itself, 

 at this time, the veins are not yet distinguishable. The 

 heart is yet not divided into chambers. The arteries going 

 out from the embryo are the umbilical-mesenteric arteries. 

 The aorta branches first, beginning with the carotid arteries, 

 and its terminal branches enter the urinary sac developing 

 at that time. In the embryo veins appear, the jugulars 

 appearing first. 



The most important changes occur in the heart: 1) it 

 gets displaced backwards; 2) as a result of its ends coming 

 close together, the heart protrudes downwards more, moving 

 between the abdominal plates; 3) it bends increasingly to 

 the right; and 4) it begins dividing into chambers. In the 

 venous part of the heart, two lateral widenings of the 

 anterior part appear. These are the primordia of the auricles, 

 actually ears, but the cavity of the auricles is not yet 

 divided. Where the common venous stem contacts the digestive 

 canal on the sides of the vein, pyramid -shaped hollow projec- 

 tions grow and acquire a leaf -shaped form enveloping the vein. 



Turning to the development of the digestive canal, Baer 

 objected to Wolff's and Pander's idea that "each part of the 

 digestive canal acquires its individuality in the formative 

 process and ... the stomach forms, the duodenum and so on." 

 In contrast, he considered that the intestine "in the beginning 

 becomes separated from all the other body by its common 

 individuality; however, it still remains homogeneous in all 

 its extension, and only later are the differences in its 

 individual parts observed" (I 5r, p. 103 (60)). 



Baer further described how the vascular layer of the 

 digestive canal becomes swollen and gives rise during the third 

 day to the lungs, liver, the pancreas, caecum and urinary sac, 

 in whose formation the protrusions of the mucous sheets also 



323 



