DEVELOPMENT OF THIRD DAY 



495 



vessels are developed later, and a superficial layer (Fig. 291, a), of epi- 

 thelium, continuous with the epithelial lining of the peritoneal cavity. 

 The withdrawal of the anterior part of the fore-gut from the notochord 

 is slight, as little or no mesentery is developed in that region. 



It is interesting to note here that the oesophagus has its lumen 

 closed for almost its entire length during the sixth day, only to reopen 

 from the posterior region anteriorly again in about two days by the epi- 

 thelial tube growing rapidly. This latter grows in a circular direction 

 on account of the outer pressure. 



The portion of the intestinal tract immediately posterior to the 

 oesophagus becomes dilated on this day to form the stomach. This is 

 followed posteriorly with a short region recognized as the duodenum 

 because the beginnings of the liver and pancreas can be observed. 

 Mesenchymal cells gather about the oesophagus and stomach from which 

 their muscular and connective tissue coats will be derived. 



There will be seen a small pitting in of the ectoderm to meet the 

 underlying entoderm where the anal opening is to appear. However, 

 this posterior opening does not open into the digestive tract until about 

 the fifteenth day of incubation. The indenture which is to become the 

 anal opening is called the proctodaeum. 



The digestive tract is almost straight until the sixth day, when the 

 various loops form and the gizzard develops as a thick-walled outgrowth 

 from the end of the stomach. 



THE LUNGS 



Two small hollow outgrowths from the 

 ventral side of the oesophagus near its anterior 

 end are seen on this day, the oesophagus itself be- 

 coming constricted at the point of outgrowth. 



These constrictions form two divisions, the 

 more dorsal being the oesophagus and the ventral 

 portion the trachea. At the point where oesopha- 

 gus and tracheae are continuous, the glottis will 

 be formed. 



The trachea grows caudad and bifurcates to 

 form pairs of lung-buds. These lung-buds extend 

 outward into the surrounding mesenchyme lying 

 on either side of the midline. The splanchnic 

 mesoderm is pushed ahead of the growing lung- 

 buds until it covers them and forms their outer 

 investment layer, or pleural covering. The ento- 

 derm of the intestinal tract from which the 

 trachea evaginated, forms the entire lining of 

 trachea, bronchi,, and all air-chambers in the 

 adult lungs. The connective tissue stroma of the 



Fig. 292. 



Ventral view of lungs and 

 air-sacs of 12 day chick 

 embryo. at, anterior in- 

 termediate sac ; a, abdominal 

 sac ; c, cervical sac ; I, lat- 

 eral part of interclavicular 

 sac ; lu, lung ; m, mesial 

 part of interclavicular sac ; 

 oe, oesophagus ; p, posterior 

 sac; t, trachea. (From 

 Kingsley after Locy and 

 Larsell. ) 



