J2o THE AMPHIOXUS. 



expression in the further development. We will get 

 a more exact view of this in the stage with nine 

 mesoblastic somites, where it is already quite distinct. 



If the embrj'-o be considered from the side, and the 

 microscope be first directed to the mesoblastic somite 

 boundaries of the left side, and next, to those of the 

 right side of the body, it may then be noticed that 

 they do not cover one another, but that those of the 

 right side come to lie somewhat further back than 

 those of the left. This condition may be most clearly 

 represented through drawing by means of the Camera 

 lucid a. In the same way this movement may be 

 indicated by a consideration of the embryo from the 

 back (Fig. E2). This movement advances gradually, 

 in the first stages of the next period of development, 

 so far forward that it extends as far as half a 

 mesoblastic somite. We see, then, the first mesoblastic 

 somite boundary of the right side fall nearly between 

 the first and second of the left side, the movement of 

 the first mesoblastic somite being not quite so impor- 

 tant as that of the rest. Then we see the second 

 between the second and third, the third between the 

 third and fourth, and so on (Fig. 54). At the posterior 

 end, where the latest mesoblastic somites lie, the move- 



