portions of the intestine, the digestive canal remains 

 straight and does not show a structural difference along 

 its length. For this stage of development of the intestine, 

 Baer considered acceptable Wolff's term "primitive intestine." 

 Along its length, it consists of two layers, the internal 

 mucous and the external vascular; above the intestine, the 

 vascular layer extends towards the vertebral column. Between 

 the mesentery plates, there remains a space which Baer 

 named the mesenteric aperture. "This aperture," Baer 

 remarked "represents what Wolff named the fistula intestinalis, 

 because he did not distinguish it from the aperture of the 

 intestine. There is his main mistake" (II, 7s fn.p. 164 (123)) 



The intestine at the described stage of development 

 represents a primitive organ not only for all the digestive 

 apparatus, but also for the respiratory and some urinary and 

 sexual organs. The most anterior part of the digestive canal, 

 the gullet, after jaw development, is directed downwards to 

 form the mouth cavity, from which the nose cavities are 

 separated by the palate. In the posterior part of the gullet 

 cavity at the third day, three pairs of slits appear which, 

 as in fish, should be named the branchiate slits. On the 

 third or fourth day the first slit usually becomes covered, 

 and posteriorly the fourth is formed. On the fifth or sixth 

 day, as a rule, the other slits are closed. The portions of 

 the gullet walls between the slits are called branchiate 

 arches . 



The intestinal tube develops somewhat later than the 

 gullet, and the crop significantly later. The stomach at the 

 beginning is not distinguished by width from the other parts 

 of the digestive tube, but later it widens. The intestine 

 gradually elongates. The small intestine, forms many loops, 

 some of which pass through the umbilicus from the abdominal 

 cavity and then extend internally with the remnants of the 

 yolk sac, which in some birds such as nightingales never 

 disappear. The posterior part of the intestine (cloaca) 

 continues . 



The liver is a protrusion of the intestinal walls, which 

 forms externally and downwards two blunt hollow appendages 

 enveloping a venous stem. The bases of the protrusion 

 gradually narrow. The protrusions of the mucous layer branch 

 into the vascular layer, which is raised in the form of a 



396 



