156 THE FOURTH DAY. [CHAP. 



over the neural canal. Fresh lines of segmentation then 

 appear in the intervertebral portions, -which run in such a 

 way that each ganglion is now more closely associated with the 

 vertebral portion in front of it than with that behind it, 

 though the latter sprang in part from the same original 

 protovertebra as itself. 



13. Meanwhile from the fourth to the sixth day im- 

 portant changes take place in the notochord itself 



On its first appearance the notochord was, as we have seen, 

 composed of scnnewhat radiately arranged but otherwise 

 perfectly typical mesoblast-cells. 



On the third day some of the central cells become vacuo- 

 lated, while the peripheral cells are still normal. The vacuo- 

 lated cells exhibit around the vacuole a peripheral 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 con- 

 dition is probably fluid. Tonards the end of the day the 

 notochord acquires a delicate structureless sheath which is no 

 doubt a product of its peripheral cells. 



According to His there is a cavity in the centre of the notochord on the 

 third day. We have never observed this, and it is denied by MuUer 

 {Ueber den Baa der Choi-da JDorsalis. Jenaische Zeitschrift. Bd. VI. 1871). 



On the fourth day all the cells become vacuolated with 

 the exception of a single layer of flattened cells at the peri- 

 phery ; and the vacuoles themselves become larger. At the 

 point where the nucleus lies there is generally rather more 

 protoplasm than round the remainder of the circumference 

 of the cells. 



On the sixth day all the cells are vacuolated. In each cell 

 the vacuoles have so much increased at the expense of the 

 protoplasm that only a very thin layer of the latter is left at 

 the circumference of the cell, at one part of which, where 

 there is generally more protoplasm than elsewhere, the 

 starved remains of a nucleus may generally be detected. 



Miiller {loc. cit.) considers that the cells have a membrane. This however 

 is probably merely a hardened external layer of the protoplasm ; and is stained 

 by reagents. 



Dursy {Zur EntwicMungsgeschichte des Kopfes des Menschen und der Tiokeren 

 Wirhelthiere) believes that what we have spoken of as vacuoles in the cells are 

 really intercellular spaces. So that according to his view the notochord is 

 com.posed of stellate cells with large round intercellular spaces filled with 

 transparent intercellular matter. Superficially viewed a section of the noto- 



