476 WILSON-. 



PLATE XVIII. 



[All excepting Fig. 60 from preparations.] 



Fig. 59. Rear view of first ciliated stage. Ninth division of X (13 h.). 



Fig. 60. Living embryo in the same stage (10 h.). 



Fig. 61. Postero-ventral view, after completion of the tenth division oi X (11 h.). 



Fig. 62. Ventral view of nearly the same stage, showing the relations of the 

 stomatoblasts, blastopore, mesoblasts, head-kidneys, prototroch, and the posterior 

 teloblasts (13 h.). 



Fig. 63. Similar view of later stage, after the eleventh division of X (disappear- 

 ance of the outer teloblasts), and closure of the blastopore. Extension of the head- 

 kidneys, infolding of the blastopore-region, convergence of the stomatoblasts (13 h.). 



Fig. 64. Rear view of nearly the same stage (11 h. 30 m.). 



Fig. 65. Succeeding stage, showing division, displacement, and initial separation 

 of the residual teloblasts (twelfth division of X). 



Fig. 66. Slightly later stage with symmetrical division of the products of the outer 

 teloblasts (11 h. 30 m.). 



Fig. 67. Similar view of still later stage. Separation of the residual teloblasts, 

 appearance of the rudimentary dorsal region, extension of the head-kidneys (14 h.). 



Fig. 68. Rear view of still later stage (14 h.). 



Fig. 69. Similar view of later stage (18 h.). 



Fig. 70. Postero-ventral view of the latest stage in which the residual teloblasts 

 can be distinguished. The upper part of the figure in surface-view (prototroch, 

 residual teloblasts), the lower part in optical section (entoblast, mesoblast, ventral 

 plate, pigment-plug) (14 h. 30 m.). 



