Development of the Third Day 203 



dinal veins. Through the Cuvlerian 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. 



