i;8 THE AMPH1OXUS. 



Fig. 113. Transverse section through the anterior end of 

 the neural plate, notochord completely differentiated, but still 

 continuing to border the cavity of the mesenteron. 



Fig. 114. Transverse section somewhat further back through 

 the anterior opening of the neural canal. 



Fig. 115. Transverse section through the posterior part of 

 the first mesoblastic somite. 



Fig. 116. Transverse section from the middle of the body. 



Fig. 117. Transverse section from the posterior third of the 

 body It passes through on the left hand the cavity of a somite, 

 on the right the boundary of a somite, and this is to be ex- 

 plained by the want of symmetry of these boundaries. 



Fig. 118. Transverse section from the region of the ninth 

 mesoblastic somite, whose cavity still communicates with that 

 of the archenteron. The notochord is in this region not yet 

 separated, the notochord slit being distinct. 



Fig. 119. Transverse section through the region of the un- 

 differentiated mesoblast folds. In the middle is the open 

 notochord fold. If this section be compared with that from the 

 corresponding differentiated region of an earlier stage, Fig. 92 

 and 103, the shortening of the development can be clearly seen. 

 Compare also Fig 141. 



Figs. 120-124. Transverse sections from embryoswith 

 10 mesoblastic somites. 



Fig. 120. Transverse section through the most anterior end 

 of the body. ISTotochord differentiated, but still bounding the 

 cavity of the mesenteron. 



Figs. 121 and 122. Succeeding sections through the neuro- 

 pcre. 



Fig. 123. Transverse section from the posterior part of the 

 first mesoblastic somite, 



Fig. 124. Transverse section from a second series of sections, 

 from the middle of the body. 



