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Development of the Third Day 203 



dinal veins. Through the Cuvierian veins or 

 ducts the blood passes into the meatus 

 venosus and thence into the heart. 



The alimentary canal. By the rapid folding 

 off of the embryo from the yolk, during this 

 day, the digestive tract becomes enclosed for 

 the greater part of its length. As a matter of 

 convenience, it is sometimes divided into three 

 regions : the anterior end that is completely 

 enclosed, having not only roof and sides, but 

 also a floor, is known as the fore-gut ; the 

 middle region that is still open to the yolk, 

 and consequently has roof and sides, but no 

 floor, is the mid-gut ; and the posterior region 

 which, like the fore-gut, is completely enclosed, 

 is the hind-gut. 



As the closing in of the digestive tract con- 

 tinues, the fore- and hind-guts, of course, in- 

 crease in length at the expense of the mid-gut 

 until the seventh day, when the opening to 

 the yolk is reduced to such a narrow opening 

 that the mid-gut may be considered to have 

 disappeared. At the end of the third day, the 

 fore-gut about corresponds to what will be the 

 oesophagus and stomach ; the hind-gut will be 

 the large intestine ; and the mid-gut will form 

 the small intestine. 



