mound. On the subsequent formation of the liver, its ducts 

 and gall bladder appear. The enlarging mass of the vascular 

 layer forms the parenchyma of the liver, and the vein which 

 is jammed between the liver rudiments branches in the 

 parenchyma as the portal vessels. Similarly, but without 

 that much close contact with the vessels, the pancreas develops 

 Baer wrongly considered that it develops as a single structure. 



All the respiratory apparatus develops by protrusion 

 from the digestive tube. Directly behind the last branchiate 

 slit, on the eighth day a pair of small hollow elevations 

 appear; these are converted into sacs along a common narrow 

 base. These sacs represent the lung rudiments from whose 

 stem-like base the respiratory tube develops. Subsequently 

 the lungs form a system of branching tubes, and the air sacs 

 develop to penetrate the entire cavity of the body and bones. 



Histological differentiation of the vascular system 

 Baer described as follows: Blood forms earlier than the 

 vessels as a result of the dilution or thinning out of the 

 previously compact or firm parts. This occurs only in the 

 vascular layer. The fluid is initially colorless, then it 

 becomes yellow and finally it becomes red blood. Under the 

 effect of the movement of blood, the permanent passages form 

 and soon acquire firmer walls. 



The process of morphological differentiation in the 

 vascular system Baer divided into four periods, each 

 involving small changes which prepare for the subsequent 

 period. The first period, in which the vascular system 

 appears, continues during the first two days of incubation. 

 On the second day, between the anterior ends of the ventral 

 plates, two elongated granular masses appear forming a 

 figure called the cardiac canal. Proceeding from it 

 anteriorly are two vascular arches, which Baer correctly 

 noted are only like veins at first; Wolff was incorrect 

 when he considered that all the vessels of the first arch 

 are veins. These arches soon become less detectable, then 

 the second pair of arches is formed, later the third, and 

 finally, between the second and third days the fourth pair 

 is formed. The arches of each side pour into canals (roots 

 of the aorta) above the gullet, which are fused into the 

 impaired aorta passing under the vertebral column; its 

 branches give rise to the arteries of the yellow sac. 



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