The third stage of the third period of development 

 C§ 11) covers days 11 through 13. The yolk sac starts to 

 diminish as a result of yolk consumption. The vascular area 

 occupies nearly the entire surface of the yolk and forms 

 wrinkled folds immersed in the yolk mass. Baer considered 

 these folds analogous to the intestinal folds of the lower 

 vertebrates; they play the role of intestinal fibers. The 

 urinary sac surrounds the yolk and the amnion. Hence, its 

 opposing borders coincide and accrete together. The internal 

 wall of the allantoides protrude into its cavity in the form 

 of a fold-structure rich in vessels, sometimes called the 

 chorion. The fluid of the allantoides contain flake-shaped 

 lungs of the urinary precipitates. In the walls of the amnion, 

 narrow vessels are distinguishable. The embryo moves more 

 actively. The beak and the claws become horny. 



Through the umbilical orifice, a twisted loop of the 

 intestine hangs over, its length at that time significantly 

 increasing. The abdominal plates have almost grown to the 

 umbilicus, leaving around it an elliptical area covered only 

 by the peritoneal membrane. From the substance of the 

 abdominal plates themselves, cartilage, muscles and nerves 

 are already formed. Ossification of the skeleton progresses 

 quickly; however, Baer could not determine regularity of this 

 process. The points of ossification in the vertebrae lie inside 

 their bodies. The narrowing of the spinal cord relates to the 

 ossification of the spinal column, which occurs backwards from 

 the anterior end. 



The organs of digestion intensively show the processes of 

 differentiation. In the wall of the esophagus, longitudinal 

 folds appear; the wall of the crop thickens and is accompanied 

 with mucous glands. From the right of the muscular stomach 

 (gizzard) the duodenum proceeds, and the loop envelops the 

 pancreas. The growing liver pushes the remaining part of the 

 intestine backwards and downwards, as a result of which the 

 small intestine moves into the umbilical orifice. The gall 

 bladder assumes a gray color, while the bile penetrates the 

 stomach and the duodenum. The cloaca acquires a folded 

 internal surface and is joined by the stem of the urinary sac. 



The lungs firmly join the chest cavity, with deep marks 

 of the ribs noticeable. The surface of the lungs begins as 

 broom-shaped or velvety, because the terminal tubules hang 



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