FIFTH PERIOD OF DEVELOPMENT. 165 



The very faintly marked boundaries of the meso- 

 blastic somites are only to be followed as far as the 

 region of the ventral edge of the iiotochord. Further 

 in a ventral direction, the partitions are formed back- 

 wards, as was already explained, and the body cavity 

 is not there divided into segmental divisions. It 

 is however no longer formed of open hollows, since 

 the layers are here in immediate contact. It can 

 however be seen that they do not grow together, 

 since treatment of various kinds, pressure, and appli- 

 cation of reagents, causes the outer surface, with its 

 thin mesoblast layers, to separate from the inner 

 layers. The ventral blood vessel, and its continuation 

 on the right side, reaching up to the club-shaped 

 gland, begins slow contractions, whose direction is 

 from back to front. The notochord shows a more 

 strongly marked form of the vacuoles and the noto- 

 chord plates. 



The absorption of the yolk granules is concluded 

 with this period. All the tissues are formed of proto- 

 plasm, which is clear as glass and transparent. 



