208 THE FOURTH DAY. [CHAP. 



tion, but having the ganglia lodged on its exterior at 

 intervals^ the neural canal is covered in with a series of 

 cartilaginous arches, connected to each other by ordinary 

 mesoblastic tissue only, but passing at their bases di- 

 rectly into the cartilaginous tube around the notochord. 



By a process of histological differentiation the carti- 

 laginous tube is divided into vertebral and interverte- 

 bral portions, the vertebral portions corresponding to 

 the arches over the neural canal. Fresh lines of seg- 

 mentation then appear in the intervertebral portions, 

 dividing each of them into two parts, of which one at- 

 taches itself to the vertebra in front and the other to 

 the vertebra behind. 



The notochord. Meanwhile from the fourth to the 

 sixth day important changes take place in the notochord 

 itself. 



On its first appearance the notochord was, as we 

 have seen, composed of somewhat radiately arranged 

 but otherwise perfectly typical mesoblast-cells. 



On the third day some of the central cells become 

 vacuolated, while the peripheral cells are still normal. 

 The vacuolated cells exhibit around the vacuole a peri- 

 pheral layer of granular protoplasm in which the nucleus 

 lies embedded, whilst the vacuoles themselves are filled 

 with a perfectly clear and transparent material, which 

 in an unaltered condition is probably fluid. Towards 

 the end of the day the notochord acquires a delicate 

 structureless sheath which is no doubt a product of its 

 peripheral cells. 



On the fourth day all the cells become vacuolated 

 with the exception of a single layer of flattened cells at 

 the periphery. The vacuoles go on enlarging until 



