EXPLANATION OF FIGURES ON PLATES. 171 



Fig. 49. The same stage, as seen from the dorsal surface. 

 The dorsal cell series of the notochord foundation is drawn in, 

 the neural canal is only indicated. 



Fig. 50. Stage with nine mesoblastic somites in optical longi- 

 tudinal section. Mesoblast formations and anterior diverti- 

 culurn of the archenteron are drawn in. 



Fig. 51. The same stage on a lateral view, with especial 

 regard to the mesoblastic somites and the mesoderm fold. The 

 first, second, third, and eighth mesoblastic somites are repre- 

 sented in the optical longitudinal section. The cells forming 

 the muscular system, which are in contact with the notochord, 

 are only indicated here. In the fifth, sixth, and seventh somites, 

 these cells are focussed. The fourth and ninth somite, as well 

 as the undifferentiated mesoblast fold, are represented as seen 

 from the upper surface. 



Fig. 52. The same stage, as seen from the dorsal surface. 

 The notochord is drawn in section; the neural canal which runs 

 over it is only indicated by shading. 



Fig. 53. The same stage as seen from the ventral side. In 

 the region of the anterior diverticula of the archenteron the 

 course of the archenteron is indicated by punctuated lines, as 

 may be seen by raising the microscope higher, and so getting 

 nearer to the ventral surface. 



PLATE Y. 



Figures 6lA, 62, and 63A, are drawn from the living object ; 

 all the others from preparations. 



Fig. 54. Stage with thirteen mesoblast somites; optical sec- 

 tion. Both diverticula of the archenteron are drawn in. The 

 mesoblastic somite boundaries of the left side are given in 

 continuous lines ; those of the right side are punctuated. 



Fig. 55. The same stage, as seen from the right, to show the 



