174 THE AMPHIOXUS. 



the separation of the individual parts from one 

 another appear black in the former, while in the 

 latter they show clear and bright. 



All the sections of a series are recognisable as 

 belonging together, by abbreviated designation of 

 the stage (i.e. the number of the mesoblastic somites 

 of the latter), as well as by a series of letters. 

 Where the representations of a stage are taken 

 from two different series of sections, that is from, 

 different individuals (for instance, Plates IX., XI., 

 US.), this may be seen from the index given with 

 the letters. 



PLATE VII. 



Fig. 71. Transverse section from the middle of the body, 

 from an embryo of the stage of Figs. 33, 34. 



Fig. 72. Transverse section from the middle of the body 

 from an embryo between the stage of Fig. 33 and that of 34. 



Figs. 73-76. Transverse sections from an embryo 

 with the first mesoblastic somite. Stage of Figs. 

 35,36. 



Fig. 73. Section immediately in front of the region of the 

 mesoblastic somite. 



Fig. 74. Section through the region of the mesoblast'e 

 somite. 



Fig. 75. Section from the middle of the body. 



Fig. 76. Section from the posterior third of the body. 



Figs. 77, 78. Transverse sections of an embryo in 

 which the second mesoblastic somite is in forma- 

 tion. Stage of Fig. 37. 



Fig. 77. Section through the "region of the first mesoblastic 

 somite. 



Fig. 78. Section from the posterior quarter of the body. 



