FOURTH PERIOD OF DEVELOPMENT. 143 



The different destiny of the vacuoles is furthermore 

 of considerable importance. "While in the dorsal and 

 ventral cell series they keep a round or somewhat 

 irregular form, increase but slightly, and thereby 

 become fainter and decrease in number, in places 

 even entirely vanishing, those of the two middle 

 series of cells come into continually sharper promin- 

 ence, increase considerably, and experience a charac- 

 teristic change of form (Figs. 54, 60, G!A). 



In consequence of the increase, these vacuoles do 

 not retain their round form, but they appear longi- 

 tudinal, both on the side and back view of the embryo; 

 it is only on transverse sections that their outline is 

 round. They are thus flattened in the direction of 

 the longitudinal axis of the embryo (Figs. 54, 60). 

 While (on a side view of the embryo) they become 

 continually higher, they are moved in such a manner 

 forwards on to one another that they form one single 

 series. The middle cells of the notochord, which 

 contain the vacuoles, have also naturally to do with 

 this movement. Whereas these cells were originally 

 to be found in two series, they form later on by 

 moving into one another merely one (Figs. 60, 61). 

 The notocLord consists now of three cell series, a 



