OUTLINE FOR LABORATORY STUDY OF THE CHICK 337 



At this age it is of interest to study the systems of organs. Ob- 

 serve the manner in which the splanchno-pleure folds to form the 

 walls of the intestine. Note the commencement of the mesentery. 

 Note that the foremost part of the alimentary tract is formed from the 

 stomodeal invagination of the ectoderm. The hypophysis is formed 

 from a dorsal outgrowth of this. Note its relation to the brain. 



The following structures are formed from outgrowths of the 

 ectoderm at this stage: 



First, the visceral pouches. 



Second, the median rudiment of the thyroid. This is an out- 

 growth from the pharynx between the two hyoid arches. 



Third, the rudimentary lungs, which develop in a pair from a 

 median ventral diverticulum of the alimentary tract, just behind the 

 last visceral pouch. The esophagus is just posterior to this. The 

 esophagus, very short at this stage, is continuous with a slightly 

 wider part that develops into the stomach. 



Fourth, the first liver diverticulum, and, at a short distance 

 posterior to this, the second liver diverticulum. 



Fifth, the pancreas is at a point where the intestine opens ven- 

 trally. It first appears as a slight thickening of the dorsal angle of 

 the intestine. 



Sixth, the ventral wall of the hind-gut forms a wide evagination. 



Seventh, the beginning of the allantois. 



After the whole series of transverse sections have been studied and 

 drawn, construct a longitudinal section of the fetus, including a 

 reconstruction of the alimentary tract. 



At this stage the heart is a simple tube. The following divisions 

 are distinctly visible: the auricular portion, the ventricular portion, 

 the sinus venosus, and the bulbus arteriosus. The union of the two 

 ductus Cuvieri and the ductus venosus form the sinus venosus. The 

 two ductus Cuvieri are formed by the union of the anterior and the 

 posterior cardinal veins. The ductus venosus is formed by the 

 union of the small right and the large left vitelline vein. These 

 latter veins return the blood from the yolk sac. The sinus venosus 

 empties into the single auricle above which it is located. The 

 single auricle is later divided into two chambers, the right and the 

 left auricle. At this stage it is widest in the lateral direction. 

 The auricle empties directly into the ventricle. The ventricle lies 

 ventrally and behind the auricle. This location is due to the bend- 

 ing of the heart at this stage of development. Its hindmost portion 



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