FIRST HALF OF SECOND DAY 475 



sen's node, an opening remains, rhomboidal in shape. This is the rhom- 

 boid sinus. 



In lower forms, there is an opening from the neural canal into the 

 digestive tract known as the neurenteric canal, or posterior neuropore, 

 at the point where the blastopore does not close until after it is sur- 

 rounded by the neural folds. In the chick the primitive pit represents 

 this region of the neurenteric canal. 



Shortly after the twenty-seventh hour period, and as soon as the 

 caudal end of the chick can be definitely outlined, the primitive streak 

 disappears entirely. 



LENGTHENING OF THE FORE^GUT 



The crescent-shaped margin of the anterior intestinal portal grows 

 more and more caudad, first because the margin from each side grows 

 toward the midline to fuse with the other side, thus lengthening the fore- 

 gut by adding to its floor, and pushing the crescentic margin caudad ; 

 and second, all structures anterior to the anterior intestinal portal are 

 elongating so rapidly that the portal is bound to lie further and further 

 caudad from the cephalic end of the embryo. 



These two processes together cause the space between the sub- 

 cephalic pocket and the margin of the anterior intestinal portal to be- 

 come elongated, and it is in this enlarging space that the pericardial por- 

 tions of the coelom extend, and in which the heart comes to lie. 



