176 THE AMFHIOXUS. 



Fig. 89. Section through the region of the third mesoblastic 

 somite. 



Fig. 90. Section through the region of the fourth mesoblastic 

 somite - 



Fig. 91. Section taken somewhat obliquely, on the left 

 passing through the fifth mesoblastic somite, which is in pro- 

 cess of formation, on the right the undifFerentiated mesoblast 

 fold. 



Fig. 92. Section from the posterior end of the body. (There 

 follow two that are similar in the series of sections.) 



Figs. 93,94. Transverse sections from an embryo 

 with five well-formed mesoblastic somites. 



Fig. 93. Transverse section passing through the anterior 

 part of the first mesoblastic somite. 



Fig. 94. Transverse section passing through the posterior 

 half of the first mesoblastic somite. 



Figs. 95-103. Transverse sections from an em- 

 bryowith six mesoblastic somites. 



Fig. 95. Transverse section through the anterior opening of 

 the neural canal. The continuations of the first mesoblastic 

 somites have advanced to this point. 



Fig. 96. Succeeding section through the region of the first 

 mesoblastic somite. Notochord fold open. 



Fig. 97. Succeeding section through the region of the first 

 mesoblastic somite. Notochord slit closed. 



Fig. 98. Succeeding section from the region of the first 

 mesoblastic somite. 



Fig. 99. Section through the posterior end of the first meso- 

 blastic somite ; notochord slit having disappeared. 



Fig. 100. Section from the region of the fourth mesoblastic 

 somite. 



Fig. 101. Section from the region of the fifth mesoblastic 

 somite. 



