Development of the Third Day 165 



empties into the left vitelline vein ; and in any 

 case the left vessel is the larger, if two are 

 present. The blood that flows backwards in 

 each half of the sinus terminalis empties into 

 a single posterior vessel, and through that is 

 brought back to the left vitelline vein (Fig. 

 56). This posterior vessel, bringing blood 

 from the hinder half of the sinus terminalis, 

 was not present, it will be remembered, during 

 the second day. 



The folding off of the embryo from the yolk 

 makes great progress, during this day, so that 

 it is now more clearly outlined by the deep- 

 ening of the head-, tail-, and side-folds, and is a 

 tubular body or sac (embryo-sac) connected 

 with the yolk-sac by a sort of wide stalk. By 

 examining Fig. 38, it will be seen that this 

 stalk is a double tube ; the inner tube or stalk 

 is known as the splanchnic stalk, and is con- 

 tinuous with the now clearly defined digestive 

 canal : the outer tube or stalk is the somatic 

 stalk, and its cavity is continuous with the 

 body-cavity, while its walls are a continuation 

 of the somatopleure (study carefully Figs. 37 

 and 38). 



A remarkable change in position takes place 

 during this day. Up to the close of the second 



