EXPLANATION OF FIGURES ON PLATES. 173 



lateral view from the right side. Mouth and the first meso- 

 blastic somite boundaries of the left side indicated with punc- 

 tuated lines. Enlargement - 6 I a . 



Fig. 66. Posterior end of a larva, in which the second gill- 

 slit has just perforated, from the living object. Enlargement 

 -j-. The mesoblastic somite boundaries of the right side of the 

 body indicated with punctuated lines. The pole-cell of the 

 mesoblast still distinguishable. The ventral wall is, by reason 

 of the pressure of the cover-slip, slightly removed from the 

 ventral contractile hlood vessel. 



Fig. 67. Larva with mouth and first gill-slit, from an osmic- 

 picro-carmin preparation. Enlargement ^\ Q -. 



Fig. 68. Ventral region of such a larva from the living 

 object. The epiblast (Ect.) has, owing to the pressure of the 

 cover-slip, become removed from the ventral blood vessel 

 adhering to the alimentary canal. Enlargement ^{^-. 



Fig. 69. Anterior end of such a larva from a preparation. 

 The lateral masses of fibrilla (N. F.) of the neural canal do not 

 reach as far as its anterior end. Enlargement - T -. 



Fig. 70. A section of such a larva from a preparation. In 

 region A the notochord is represented in optical longitudinal 

 section. In region B the microscope is directed to the muscle 

 fibrilla, in region C to the protoplasmic bodies of the muscle 

 cells. Enlargement -^f^. 



Plates VII., VIII., and IX. contain representations 

 of sections. The enlargement is, with the exception 

 of Figs. Ill and 151, universally the same (^-). The 

 drawings are as far as possible true to nature, 

 though the carmin staining is represented by a 

 darker colour, and the cell granules in the litho- 

 graphy are from reasons of economy kept engraved. 

 Further, the drawing differs from the microscopic 

 representation in that the cell boundaries and 



