Development of the First Day 127 



derived ; this sheet of mesoblast extends until 

 it passes the boundaries of the area pellu- 

 cida, and forms a middle layer in the inner 

 zone of the area opaca. This zone of the 

 area opaca which contains the mesoblast, and 

 which immediately surrounds the area pellu- 

 cida, is known as the vascular area, because 

 it is in it that the blood vessels that absorb 

 the yolk, for the growth of the embryo, are 

 formed (Fig. 80, ar. vase). 



The cells of the mesoblast are usually not 

 closely packed together, and may generally 

 be recognized by their angular or stellate 

 form (Figs. 42 and 48). 



At about the time of the formation of the 

 primitive streak and the differentiation of 

 the mesoblast, the entoblast, in front of the 

 primitive streak, becomes thickened to form 

 a longitudinal axis or rod of cells, the noto- 

 chord (Fig. 42, ch). The notochord remains 

 for some time attached to the entoblast from 

 which it is derived, but it later separates 

 from this, and forms a distinct and separate 

 rod, similar to that which we have already 

 seen in the frog. 



While the entoblast has been forming 

 (about the fifteenth to the twentieth hour) the 



