i So THE AMPHIOXUS. 



Fig. 138. Succeeding somite, on the right side the somite 

 boundary is passed through. 



Fig. 139. Transverse section through the thirteenth meso- 

 blastic somite. 



Fig. 140. Transverse section through the most posterior, i.e. 

 fourteenth, mesoblastic somite. Notochord not fully separated. 



Fig. 141. Transverse section through the undifferentiated 

 mesoblast folds which are still in open communication with the 

 lumen of the mesenteron. 



Fig. 142. Transverse section immediately in front of the 

 neurenteric canal. 



Fig. 143. Transverse section passing through the opening of 

 the neurenteric canal into the alimentary canal. 



Fig. 144. Succeeding transverse section through the most 

 posterior point of the body. 



Figs. 145-151. Transverse sections through a larva 

 with mouth and first gill-slit. 



Fig. 145. Transverse section immediately behind the anterior 

 opening of the neural canal. 



Fig. 146. Transverse section through the anterior edge of 

 the opening of the mouth. 



Fig. 147. Transverse section through the posterior part of 

 the opening of the mouth. 



Fig. 148. Transverse section immediately in front of the 

 first gill-slit. 



Fig. 149. Transverse section through the gill-slit. 



Fig. 150. Transverse section through the middle of the body. 



Fig. 151. Transverse section through the same region more 

 enlarged (*p). N. F. nerve fibrilla, M.F. muscle fibrilla. 



