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 



FIG. 73. TRANSVERSE SECTION THROUGH THE DORSAL REGION 

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



Am, amnion. Cam, am nio/ic cavity. CO, chorion. IG, 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 



