172 THE AMPHIOXUS. 



foundation of the club-shaped gland and the diverticulum on 

 the right side, which is still in connection with the archen- 

 teron. 



Fig. 56. The same stage, as seen from the dorsal surface. 

 The notochord is drawn in, and the swelling of the brain and 

 the central canal are also indicated. 



Fig. 57. The same stage, as seen from the ventral side. 



Fig. 58. Some isolated muscle cells of the same stage, on a 

 lateral view. On the left hand of the ruptured line the proto- 

 plasmic body is focussed ; on the right the fibrilla, which are 

 situated deep down. 



Fig. 59. Notochord with adhering muscle cells, as seen from 

 the dorsal surface. Isolated preparation. The vacuoles of the 

 notochord are not drawn in. 



Fig. 60. Stage with fourteen mesoblastic somites, as seen 

 from the right. The dorsal and ventral series of small vacuoles 

 is not represented in the figures drawn from preparations. 



Fig. 61. Further stage, as seen from the right. 



Fig. 6lA. The same stage, from the living object. 



Fig. 62. Anterior end of an embryo, with a still very small 

 opening of mouth, and first gill-slit. 



Fig. 63. Just such an embryo from the living object. 



Fig. 63A. Anterior end of the same stage, from the living 

 object. 



PLATE VI. 



Fig. 64. Larva with mouth and first gill-slit, as seen from 

 the left side, drawn from the living object. The first, second, 

 and third mesoblastic somite boundary of the opposite (right) 

 side of the body is indicated by punctuated lines. The gill-slit 

 situated on the right side is indicated, as seen through the body. 

 Enlargement *-.. 



Fig. 65- Just such a larva, anterior end, as seen from a right 



