Development of the Fourth Day 231 



The notochord. In connection with the 

 development of the vertebral column, a few 

 words should be said about the changes that 

 take place in the notochord. The origin of 

 the notochord, during the first day, as a longi- 

 tudinal, rod-like thickening of the entoblast 

 has been mentioned. It is at first a solid rod 



CO 



FIG. 73. TRANSVERSE SECTION THROUGH THE DORSAL REGION 

 OF AN EMBRYO OF 96 HOURS. (After Duval.) 



Am, amnion. Cam, amnioric cavity. CO, chorion. 7G, alimentary canal. 

 PPE, body cavity (external). PPI, body cavity (internal). VJ, wall of umbilical 

 stalk. VOM, vitelline vein. 



of somewhat radially arranged cells, but dur- 

 ing the third day some of the central cells 

 become vacuolated, their protoplasm collecting 

 as a thin peripheral layer, while a clear, ap- 

 parently watery material, collects in the centre. 

 In the peripheral layer of protoplasm the nu- 

 cleus is seen. 



Towards the end of the third day, a thin 

 structureless sheath is formed around the 



