VI.J THE MESOBLASTIC SOMITES. 186 



eventually spreads out over the whole body of the 

 chick. On the first half of the fourth day the vesicle is 

 still very small, and its growth is not very rapid. Its 

 mesoblast wall still remains very thick. In the latter 

 half of the day its growth becomes very rapid, and it 

 forms a very conspicuous object in a chick of that date 

 (Fig. 67, Al). At the same time its blood-vessels be- 

 come important. It receives its supply of blood from 

 two branches of the aorta known as the allantoic arte- 

 ries, and the blood is brought back from it by two allan- 

 toic veins which run along in the body walls, and after 

 uniting into a single trunk fall into the vitelline vein 

 close behind the liver. 



Mesoblast of the trunk. Coincidently with the 

 appearance of these several rudiments of important 

 organs in the more or less modified splanchnopleure- 

 folds, the solid trunk of the embryo is undergoing 

 marked changes. 



When we compare a transverse section taken through 

 say the middle of the trunk at the end of the third day 

 (Fig. 65), with a similar one of the second day (Fig. 34), 

 or even the commencement of the third day (Fig. 64), 

 we are struck with the great increase of depth (from 

 dorsal to ventral surface) in proportion to breadth. This 

 is partly due to the slope of the side walls of the body 

 having become much steeper, as a direct result of the 

 rapidly progressing folding off of the embryo from the 

 yolk-sac. But it is also brought about by the great 

 changes both of shape and structure which are taking 

 place in the mesoblastic somites, as well as by the 

 development of a mass of tissue between the notochord 

 and the hypoblast of the alimentary canal. 



