1 70 THE AMPHIOXUS. 



Fig. 40. The same stage on a lateral view. 

 Fig. 41. Stage with three mesoblastic somites, as seen from 

 the dorsal surface, from the living object. 



PLATE IV. 



All the Figures are drawn from osmic-carmin-glycerine pre- 

 parations. 



Fig. 42. Stage with two mesoblastic somites in optical longi- 

 tudinal section. Mesoblastic somites and mesoblast fold are 

 indicated. 



Fig. 43. The same stage, as seen from the dorsal surface. 

 Together with the optical front section, the mesoblastic somites 

 and mesoblast folds and the neural canal are indicated in the 

 drawing. The covering of the neural canal is ruptured in the 

 line of union owing to the influence of the reagents, just as in 

 Fig. 45. 



Fig. 44. Stage with three mesoblastic somites in optical 

 longitudinal section. The mesoblastic somites which can be 

 seen by raising the microscope are here drawn in. 



Fig. 45. The same stage, as seen from the dorsal surface. 

 Eepresentation as in Fig. 43. The region of the mesoblastic 

 somites and mesoblast fold is in the optical front section, 

 drawn on the right if the microscope be raised, on the left if it 

 be lowered. 



Fig. 46. Stage with five mesoblastic somites in optical longi- 

 tudinal section. The mesoblastic somites are drawn in. 



Fig. 47. The same stage, as seen from the dorsal surface. 

 The openings of the mesoblastic somite cavities into the cavity 

 of the archenteron are indicated, which are to be seen by 

 lowering the microscope. 



Fig. 48. Stage in which the eighth mesoblastic somite is in 

 formation, mesoblastic somites and anterior direrticula of the 

 archenteron are indicated (small individual). 



